from Day Two of
dialogue on the two great world systems

Galileo Galilei

From edition edited by Giorgio de Santillana, U. of Chicago Press, 1953

INTERLOCUTORS: Salviatus, Sagredus, Simplicius

Alviatus: The digressions of yesterday, which led us out of the path of
our principal discourse, were such and so many that I know not how I
can recover the track in which I am to proceed without your assistance.

sagredus: I do not wonder that you, who have your fancy charged
and laden both with what has been, and is to be, spoken, should find
yourself in some confusion. However, being only an auditor and having
nothing to burden my memory except such things as I have heard, I may
perhaps recover the first thread of our discourse. As far as my memory
serves me, the sum of yesterday's conference was an examination of the
principles of Ptolemy (figure 1) and Copernicus (figure 2) and which of
their opinions is the more probable and rational; the former, which af-
firms the substance of the celestial bodies to be ingenerable, incorruptible,
unalterable, impassible, and, in a word, exempt from all kind of change,
save local motion, and therefore to be a fifth, essence quite different from
this of our elementary bodies, which are generable, corruptible, alterable,
etc.; or else the latter, which, removing such differences among the parts
of the world, holds the Earth to enjoy the same perfections as the other
constitutive¹ bodies of the universe and esteems it a movable and erratic
globe, no less than the Moon, Jupiter, Venus, or any other planet. Lastly,
we made many particular parallels between the Earth and the Moon; more
with the Moon than with any other planet, because we have greater and
more sensibly detailed knowledge of it, as it is closer. Having, lastly, con-
cluded this second opinion to be more probable than the first, I should
think it best now to begin to examine whether the Earth is to be esteemed
immovable, as has been believed till now by most men, or else movable, as
some ancient philosophers and others for some time past have been of the
opinion, and, if it be movable, to enquire what kind its motion may be.²

salv.: I see already what way I am to take, but, before we offer to
proceed any further, I have to say something touching those last words
you spoke—that the opinion which holds the Earth to be endowed with
the same movements as other celestial bodies seems to be more true than
the contrary. I affirmed no such thing, as I do not intend either to state a
definite conclusion on any other of the controversial questions; but I only
intended to produce on either part those reasons and answers, arguments
and solutions, which have been hitherto thought upon by others, together
with certain others which I have stumbled upon in my long searching
thereinto, always remitting the decision to the judgment of my betters.

sagr.: I was transported unawares by my own sense of the thing, and,
believing that others ought to judge as I did, I assumed that conclusion
universal, instead of particular. I confess I have erred, and all the more in
that I do not know what Simplicius believes on this point.

simplicius: I must confess that I have been ruminating all night of what

passed yesterday and, to say the truth, I have met with many acute, new, and
plausible notions; yet I remain convinced by the authority of so many great
writers and in particular. ... I see you shaking your head, Sagredus, and
grinning to yourself, as if I had uttered some great absurdity.

sagr.: I not onlv grin but actually am ready to burst with holding

myself from laughing outright, for you have put me in mind of a very
pretty episode that I witnessed not many years since, together with some
others of my worthy friends which I could name to you.

salv.: It would be well that you told us what it was, so Simplicius may
not still think that he is the point of your laughter.                                     

sagr.: Very well. One day at his home in Venice, I found a famous
physician to whom some flocked for their studies, while others sometimes
times came thither out of curiosity to see certain bodies dissected by the
hand of a no less learned than careful and experienced anatomist. It chanced
upon that day, when I was there, that he was in search of the origin and
stem of the nerves, about which there is a famous controversy between the
Galenists and Peripatetics. The anatomist shewed how the great trunk of
nerves, departing from the brain, their root, passed by the nape of the
neck, extended themselves afterwards along the backbone, and branched
out through all the body, while only a very small filament, as fine as a
thread, went to the heart. Then he turned to a gentleman whom he knew

to be a Peripatetic philosopher, and for whose sake he had uncovered and
proved everything, and asked if he was satisfied and persuaded that the
origin of the nerves was in the brain and not in the heart. The philoso-
pher, after he had stood musing a while, answered: "You have made me see
this business so plainly and sensibly that did not the text of Aristotle³
assert the contrary, which positively affirms the nerves to proceed from
the heart, I should be constrained to confess your opinion to be true.”

simp.: I would have you know, my Masters, that this controversy about
the origin of the nerves is not yet so proved and decided, as some may
perhaps persuade themselves.

sagr.: Nor doubtless shall it ever be if it finds such contradictors; but
what you say does not at all lessen the extravagance of the answer of that
Peripatetic, who against such sensible experience did not produce other
experiments or reasons of Aristotle but his bare authority and pure ipse
dixit.

simp.: Aristotle would not have gained so great authority but for the
force of his demonstrations and the profundity of his argument. It is req-
uisite that we understand him, and not only understand him, but have so
great a familiarity with his books, that we form a perfect idea of them in
our minds, so that every saying of his may be always, as it were, present in
our memory. He did not write for the vulgar, nor is he obliged to spin out
his syllogisms in the trivial ordered method; on the contrary, using the
perturbed order, he has sometimes placed the proof of one proposition
among texts which seem to treat of quite another point. Therefore it is
necessary to be master of all that vast system and to learn how to connect
this passage with that and to combine this text with another far remote
from it; he who has thus studied him will know how to gather from his
books the demonstrations of every knowable deduction, because they con-
tain all things.                                                        

***

salv.: Let our contemplation begin therefore with this view: that,
whatever motion may be ascribed to the Earth as a whole, it is necessary
that it be to us, as partakers of it, altogether imperceptible, so long as we
have regard only to terrestrial things. On the other hand, it is equally
necessary that the same motion should appear common to all other bodies

and visible objects separated from the Earth and that therefore do not
participate in it. So that the true method of finding it, and, once found, to
know what it is, is to consider and observe if in bodies separated from the
Earth one may discover any appearance of motion which equally suits all
the rest of them. For a motion that is only seen, for example, in the Moon,
and has nothing to do with Venus or Jupiter or any other stars, cannot in
any way belong to the Earth or to any other save the Moon alone. Now
there is a most general and grand motion above all others, and it is that by
which the Sun, the Moon, the other planets, and the fixed stars, and, in a
word the whole Universe, except only the Earth, appear to move from the
east towards the west in the space of twenty-four hours; and this, at first
sight, might as well belong to the Earth alone, as, on the other hand, to all
the rest of the world, except the Earth; for the same phenomena will ap-
pear in the one case as in the other. Hence it is that Aristotle and Ptolemy,
having hit upon this consideration when they go about to prove the Earth
to be immovable, argue only against this diurnal motion; except that Aris-
totle hints something in obscure terms against another motion ascribed to
it by an Ancient,4 of whom we shall speak in its place.

sagr.: I very well perceive the necessity of your conclusion; but I meet
with a doubt from which I do not know how to free myself. It is that, since
Copernicus assigns another motion to the Earth beside the diurnal, which,
according to the rule even now laid down, ought to be imperceptible to us
but visible in all the rest of the world, I might necessarily infer either that
he has manifestly erred in assigning the Earth a motion to which there
does not appear a general correspondence in heaven or else that, if there
be such a congruity therein, Ptolemy has failed in not confuting it, as he
has done in the other.                                                  

Salv.: You have good cause for your doubts, and, when we come to
treat of the other motion, you shall see how far Copernicus excelled Ptole-
my in clearness and elevation of intellect in that he saw what the other did
not; I mean the admirable correspondence whereby this motion reflected
itself in all the other celestial bodies. But for the present we will suspend
this particular and return to our first consideration.

Beginning, then, with more general things, I will propose reasons which
seem to favour the mobility of the Earth and then the answers which
Simplicius shall make. First, if we consider only the immense magnitude

of the starry sphere compared to the smallness of the terrestrial globe, and
weigh the velocity of the motions which must in a day and night make an
entire revolution, I cannot persuade myself that there is any man who
believes it more reasonable and credible that it is the celestial sphere that
turns round, while the terrestrial globe stands still.

sagr.: If in the totality of effects which may in Nature depend upon
such like motions, there should follow in one hypothesis exactly all the
same consequences as in the other. I would esteem at first inspection that
he who should hold it more rational to make the whole Universe move, in
order to keep the Earth from moving, is less reasonable than he who being
at the top of the dome of your Cathedral in Florence, in order to behold
the city and the fields about it, should desire that the whole country might
turn round, so that he might not be put to the trouble to turn his head.
And surely the conveniences that could be drawn from this position would
have to be many and great in order to equate in my mind, and to over-
come, this absurdity in such manner as to make it more credible than the
former. But perhaps Aristotle, Ptolemy, and Simplicius must find certain
advantage therein, which they would do well to communicate to us also, if
any such there be; or else they had better declare that there neither is nor
can be any.

salv.: I have not been able, much as I have thought upon it, to find
any diversity in it, and indeed I think I have found that no such diversity
can be;⁵ I even esteem it to no purpose to seek further after it. Therefore,
observe: motion is motion and operates as motion by how much relation
it has to things which want motion; but in those things which all equally
partake of it, it operates nothing and is as if it never were. Thus the mer-
chandises with which a ship is laden move only as the ship leaves Venice
and sails by Corfu, Crete, and Cyprus for Aleppo. Venice, Corfu, and
Crete stand still, not moving with the ship. But the distance between Ven-
ice and Syria is as nothing to the chests, bales, and other parcels in respect
to the ship itself, as nothing alters between them. But if a bale of the cargo
were moved one inch from a chest, this alone would be a greater motion
for that bale in respect to the chest, and to the ship, than the whole voyage
of more than two thousand miles made by them together.

simp.: This doctrine is good, sound, and altogether Peripatetic.

salv.: I hold it to be much more ancient, and suspect that Aristotle, in
receiving it from some good school, did not fully understand it and that
therefore, having delivered it with some alteration, it has been an occasion
of confusion, owing to those who would defend whatever he says. And
when he wrote that, whatsoever moves, does move upon something im-
movable,⁶ I suppose that he equivocated and meant that, whatever moves,
moves with respect to something immovable; this proposition admits no
doubt, and the other many.

sagr.: Pray you, make no digression but proceed in the discourse you
began.

salv.: It being manifest therefore that the motion which is common to
many movables is meaningless and null in the relation of those movables
between themselves, because among themselves they make no changes
and is operative only in the relation that those movables have to other
things which want that motion; and, since we have divided the Universe
in two parts, one of which is necessarily movable and the other immov-
able, then whatever may depend upon or be required from such a motion
may as well be done by making the Earth alone move as all the rest of the
world. For the operation of such a motion consists in nothing else, save in
the relation which is between the celestial bodies as a whole and the Earth,
and this relation only exchanges its terms. Now if, for obtaining the same
effect ad unguem,7 it will be all one whether the Earth alone or the whole
Universe moves with the same motion, who would believe that Nature
(which by common consent does not work by many things what may be
done by few) has chosen to make innumerable most vast bodies move
with an inconceivable velocity, to perform that which might be done by
the moderate motion of one alone about its own centre?

SlMP.: I do not well understand how this grand motion can be as noth-
ing for the Sun, the Moon, the other planets and the innumerable multi-
tude of fixed stars. Or why you should say that it is no purpose for the Sun
to pass from one meridian to another, to rise above this horizon, to set
beneath that other, to make day and night; and I will say the same as to
the like variations made by the Moon, the other planets, and the fixed
stars themselves.

salv.: All these alterations instanced by you are nothing, except in
relation to the Earth; and, to see that this is true, just imagine the Earth

removed, and there will be no such thing in the world as the rising or the
setting of the Sun or Moon, or horizons, days or nights, nor will such a
motion cause any change between the Moon and Sun, or any other star
whatever, whether fixed or erratic. But all these changes have relation to
the Earth, and all import nothing else than that they should shew the Sun
now to China, anon to Persia, then to Egypt, Greece, France, Spain, Amer-
ica, etc. And the same for the Moon and the rest of the celestial bodies.
[2] This same effect falls out exactly in the same manner if, without troubling
so great a part of the Universe, the terrestrial globe be made to revolve in
itself. But—and this augments the difficulty when you ascribe this great
motion to heaven—you must of necessity make it contrary to the particu-
lar motion of all the spheres of the planets, each of which admittedly has
its peculiar motion from the west towards the east. Then you have to say
they are hurried to the contrary part, i.e., from east to west, by this most
furious diurnal motion; whereas, on the contrary, by having the Earth
move in itself, the contrariety of motions is taken away, and the sole mo-
tion from west to east is accommodated to all appearances and exactly
satisfies every phenomenon.

simp.: As to the contrariety of motions it would import little, for Aris-
totle demonstrates that circular motions are not contrary to one another
and that theirs cannot be truly called contrariety.

saly.: Does Aristotle demonstrate this, or does he not rather barely
affirm it, as serving for some certain design of his? If contraries be those
things that destroy one another, as he himself affirms, I do not see how
two bodies that encounter each other in a circular line should less preju-
dice each other than if they interfered in a straight line.

sagr.: Hold a little, I pray you. Tell me, Simplicius, when two knights
encounter each other, tilting in open field, or when two whole squadrons,
or two fleets at sea, make up to grapple, and are broken and sunk, do you
call these encounters contrary to each other?

simp.: Yes, we say they are contrary.

sagr.: How, then, is there no contrariety in circular motions? These
motions, being made upon the surface of the Earth or water, which are, as
you know, spherical, come to be circular. Can you tell, Simplicius, what
those circular motions can be that are not contrary to each other? They
are, if I mistake not, those of two tangent circles, where one of them being

turned makes the other move the contrary way;⁸ but, if one of them shall
be within the other, it is impossible that they should not jostle each other,
their motion being made towards different points.

salv.: But be they contrary or not contrary, these are but altercations
of words; and I know that it would be far more proper and agreeable with
Nature if we could save all with only one motion than to introduce two
that are opposite (if you will not call them contrary). Yet I do not censure
this introduction of opposites as impossible or pretend from the denial
thereof to infer a necessary demonstration but only the greater probability
of the other.

A third-reason which makes the Ptolemaic hypothesis less probable is
that it most unreasonably confounds the order that we assuredly see among
those celestial bodies of which the circumgyration is not questionable but
most certain. And that order is that, the greater a sphere, the longer is its
revolution. Thus Saturn, describing a greater circle than all the other plan-
ets, completes it in thirty years; Jupiter finishes its, that is less, in twelve
years; Mars in two; the Moon run through its, so much less than the rest,
in only a month. Nor do we see less sensibly that that one of the Medicean
Stars9 which is nearest to Jupiter makes its revolution in a very short time,
that is, in forty-two hours, or thereabouts, the next to that in three days
and a half, the third in seven days, and the most remote in sixteen. And
this rate holds well enough, nor will it alter at all when we assign the
rotation of twenty-four hours to the terrestrial globe. But, if you would
have the Earth immovable, it is necessary that, when you have passed
from the short period of the Moon to the others successively bigger, until
you come to that of Saturn, which is of thirty years, it is necessary, I say,
that you pass to another sphere incomparably greater than that and make
this accomplish an entire revolution in twenty-four hours. And this yet is
the least disorder that can follow. For if anyone should pass from the sphere
of Saturn to the starry orb, and make it so much bigger than that of Sat-
urn, as proportion would require in respect of its own very slow motion of
many thousands of years,¹⁰ then it must certainly be a jump much more
absurd to skip from this to another still bigger one, the Primum Mobile,
and to make it revolve in twenty-four hours.

But if the motion of the Earth is granted, the order of the periods will
be exactly observed, and from the very slow sphere of Saturn we come to

the fixed stars which are wholly immovable and so avoid a fourth difficulty
we would have if the starry sphere be supposed movable. That is the im-
mense disparity between the motions of those stars themselves, of which
some would come to move most swiftly in most vast circles, others most
slowly in circles very small, according as the former or the latter should be
found nearer or more remote from the Poles. This is accompanied with
still more inconveniences, because we see that those of whose motion there
is no question have been made to move all on a great circle; also, because
it seems to be an act done with no good consideration to constitute bodies
that are designed to move circularly at immense distances from the centre
and afterwards to make them move in very small circles.

And not only the magnitudes of the circles and consequently the ve-
locity of the motions of these stars shall be most different from the circles
and motions of those others, but (which shall be the fifth inconvenience)
the self-same star shall successively vary its circles and velocities. For those
which two thousand years since were in the Equator, and consequently
described very vast circles, being in our days many degrees distant, must
of necessity become slower and be reduced to move in lesser circles. And it
is not altogether impossible but that a time may come in which some of
them which in aforetime had continually moved shall be reduced by unit-
ing with the pole to a state of rest and then, after some time of cessation,
shall return to their motion again; whereas the other stars whose motion
none doubt all describe the great circle of their sphere and in that main-
tain themselves without any variation.

The absurdity is further increased (which is the sixth inconvenience),
in that no thought can comprehend what ought to be the solidity of that
immense sphere, whose depth so steadfastly holds fixed such a multitude
of stars, which are with so much concord carried about without ever chang-
ing site among themselves with so great disparity of motions. Or else,
supposing the heavens to be fluid, as we are with more reason to believe,11
so that every star wanders to and fro in it by ways of its own, what rules
shall regulate their motions, and to what purpose, so that, seen from the
Earth, they appear as though they were made by one single sphere? It is
my opinion that they might so much more easily and more conveniently
do that, by being constituted immovable, than by being made errant, by

as much as it is easier to number the blocks in the pavement of a piazza.
than a rout of boys which run up and down upon them.

Lastly, which is the seventh instance, if we attribute the diurnal motion
to the highest heaven, it must be constituted of such a force and efficacy as
to carry along with it the innumerable multitude of fixed stars, bodies of
cast magnitude far bigger than the Earth, and, moreover, all the spheres of
the planets, notwithstanding that both the first and the last by their own
nature move the contrary way.¹² And, beside all this, it must be granted
that also the element of fire and the greater part of the air are likewise
forcibly hurried along with the rest¹³ and that only the little globe of the
Earth contumaciously and pertinaciously stands unmoved against such an
impulse. This in my thinking is very difficult nor can I see how the Earth,
a suspended body equilibrated upon its centre, exposed indifferently to
either motion or rest, and environed with a liquid ambient, should not
yield also like the rest and be carried about. But we find none of these
obstacles in making the Earth move, a small body and insensible, com-
pared to the Universe, and therefore unable to offer it any violence.

sagr.: I find my fancy stirred with certain conjectures sprung as it
were of themselves, from your discourse, so that, if I want to apply myself
with attention to what follows, I must try to order them somewhat and
gather thence their true construction, if any can be made of them; and,
perhaps, proceeding by interrogation, this may help me to express myself
more easily. Therefore, I demand first of Simplicius whether he believes
that several motions may naturally apply to one and the same movable
body or else that it be requisite that its natural and proper motion be only
one.

SlMP.: To one single movable there can naturally apply but one motion
and no more; the rest all happen accidentally and by participation. Like
him who walks upon the deck of a ship, his proper motion is that of his
walk; his motion by participation, that of the ship.

sagr. : Tell me, secondly: That motion which is communicated to any
movable by participation, while it moves by itself with another motion
different from the participated, is it necessary that it should reside in some
certain subject by itself, or else can it subsist in nature alone without other
support?
simp.: Aristotle gives you an answer to all these questions and tells you
that, as of one body the motion is but one, so of one motion the body is
but one; and, consequently, that, without inherence in its subject, no
motion can either subsist or be imagined.

sagr.: I would have you tell me, in the third place, whether you be-
lieve that the Moon and the other planets and celestial bodies have their
proper motions and what they are.

simp.: They have so, and they are those according to which they run
through the zodiac; the Moon completes it in a month, the Sun in a year,
Mars in two, the starry sphere in those so many thousand. And these are
their proper, or natural, motions.

sagr.: But that motion with which I see the fixed stars, and with them
all the planets, go unitedly from east to west and return round the east
again in twenty-four hours, how does it belong to them?

simp.: It belongs to them by participation.

sagr.: Then it resides not in them; and, not residing in them or being
able to subsist without some subject in which it is resident, it must of
force be the proper and natural motion of some other sphere.

simp.: For this purpose astronomers and philosophers have found an-
other high sphere, above all the rest, without stars, to which naturally
belongs the diurnal motion; and this they call the Primum Mobile; this
carries along with it all the inferior spheres, contributing and imparting its
motion to them.¹⁴

sagr.: But if, without introducing other spheres unknown and hugely
vast, without other motions or communicated raptures, with each sphere
having its sole-and simple motion, without intermixing contrary motions,
but making all turn one way, as they should, depending as they do all
upon one principle, if we can find all things proceeding orderly and corre-
spond with most perfect harmony, why do we reject this alternative and
give our assent to those prodigious and laborious conditions?

simp.: The difficulty lies in finding out this so natural and expeditious
a way.

sagr.: In my judgment it is found. Make the Earth the Primum Mo-
bile, that is, make it turn round its own axis in twenty-four hours and in
the same direction as all the other spheres; and, without need of imparting

this same motion to any other planet or star, all shall have their risings,
settings, and all other appearances.

simp.: The business is, to be able to make the Earth move without a
thousand inconveniences.

salv.: All the inconveniences shall be removed as fast as you propound
them. The things said hitherto are only the primary and more general
inducements which give us to believe that the diurnal revolution may not
altogether without probability be applied to the Earth rather than to all
the rest of the Universe. And these inducements I put forward, not as
inviolable axioms, but as hints, which carry with them some likelihood.
But I know very well that one sole experiment or conclusive demonstra-
tion produced to the contrary suffices to overthrow these and a thousand
other probable arguments; hence it is fit not to stay here but to proceed
forward and hear what Simplicius answers and what greater probabilities
or stronger arguments he alleges on the contrary.

simp.: I will first say something in general upon all these consider-
ations together, and then I will descend to some particulars. It seems that
you base all you say upon the greater simplicity and facility of producing
the same effects. To this I reply that I am also of the same opinion if I
think in terms of my own not only finite but feeble power; but, with
respect to the strength of the Mover, which is infinite, it is no less easy to
move the Universe than the Earth, yea, than a straw. And if his power be
infinite, why should he not rather exercise a great part of it than a smaller?
Therefore, I hold that your discourse in general is not convincing.

salv.: If I had at any time said that the Universe did not move for want
of power in the Mover, I would have erred, and your reproof would have
been seasonable; and I grant you that to an infinite power it is as easy to
move a hundred thousand as one. But what I did say does not concern the
Mover; it refers only to the bodies, and, in them, not only to their resis-
tance, which doubtless is less in the Earth than in the Universe, but to the
many other particulars just considered. As to what you say in the next
place, that of an infinite power it is better to exercise a great part than a
small, I answer that one part of the infinite is not greater than another, if
both are finite. Nor can it be said that, of the infinite number, a hundred
thousand is a greater part than two, though the first be fifty thousand
times greater than the second. If a finite power is necessary to move the

Universe, though it is very great in comparison to that which suffices to
move the Earth alone, yet it does not involve a greater part of the infinite
power, nor is that part less infinite which remains unemployed. So that to
apply to a particular effect a little more or a little less power means noth-
ing; besides that, the operation of such virtue has not for its bound or end
the diurnal motion only. But there are several other motions in the World
that we know of, and there may be many others that are unknown to us.
Therefore, with respect to the bodies, and granting it as out of question
that it is a shorter and easier way to move the Earth than the Universe,
and, moreover, having an eye to the many other abbreviations and apt-
nesses that are to be obtained only this way, a most true maxim of Aristo-
tle teaches us that frustra fit per plura quod potest fieri per pauciora, "that is
done in vain by many means which may be done with fewer. This ren-
ders it more probable that the diurnal motion belongs to the Earth alone
than to the whole Universe, with the sole exception of the Earth.

simp.: In quoting that axiom, you have omitted a small clause, which
is as important as all the rest; that is, the words aeque bene, "equally well."
We ought therefore to examine whether this hypothesis satisfies equally
well all particulars as the other.

saly.: Whether both these positions work "equally well" may be de-
cided from the particular examination of the appearances; for hitherto we
have discoursed, and will continue to argue, ex hypothesi, namely, suppos-
ing that, as to the satisfaction of the appearances, both the assumptions
are equally accommodated. As to the clause which you say was omitted by
me, I have more reason to suspect that it was superfluously inserted by
you. For the expression "equally well" is a relative that necessarily requires
two terms at least, for a thing cannot have relation to itself; nor do we say,
for example, that rest is equally good as rest. When we say, "That is done
in vain by many means which may be done with fewer," we mean that
that which is to be done ought to be the same thing, not two different
ones. Now the same thing cannot be said to be done as well as itself;
therefore, the addition of the phrase aeque bene is superfluous and a rela-
tion that has but one term only.

sagr.: Unless you will have the same befall us as did yesterday, let us
return to our matter in hand; and let Simplicius begin to produce these

difficulties that seem, in his opinion, to thwart this new disposition of the
world.

simp.: That disposition is not new but very old; and that you may see
it is so, Aristotle refutes it; and his refutations are these:¹⁵ "First, if the
Earth moves either in itself about its own centre or in a circle around the
centre, it is necessary that that motion be violent, for it is not its natural
motion; for, if it were, each of its parts would partake thereof; but each of
them moves in a straight line towards the centre. It being therefore violent
and preternatural, it could never be perpetual. But the order of the world
is perpetual. Therefore, etc. Secondly, all the other movables that move
circularly seem to lag behind and to move with more than one motion,
the Primum Mobile excepted; hence it would be necessary that the Earth
also move with two motions. And, if that should be so, it would inevitably
follow that mutations should take place in the fixed stars, which, however,
are not perceived; nay, without any variation, the same stars always rise
from the same places and set in the same places. Thirdly, the motion of the
parts is the same with that of the whole and naturally tends towards the
centre of the Universe; and for the same cause rests, being arrived thither."
He thereupon moves the question whether the motion of the parts has a
tendency towards the centre of the Universe or to the centre of the Earth,
and he concludes that it goes by proper instinct to the centre of the Uni-
verse and per accidens to that of the Earth; of this point, we largely dis-
coursed yesterday. He lastly confirms the same with a fourth argument
taken from the experiment of heavy bodies which, falling from on high,  
descend perpendicularly to the Earth's surface; and in the same manner
projectiles shot perpendicularly upwards return perpendicularly down
again.

All these arguments prove their motion to be towards the centre of the
Earth, which without moving at all waits for them and receives them. He
intimates in the last place that the astronomers allege other reasons in
confirmation of the Earth's being immovable in the centre of the Uni-
verse, and instances only in one of them, to wit, that all the phenomena or
appearances that are seen in the motions of the stars perfectly agree with
the position of the Earth in the centre; this would not be so were the Earth
located otherwise. I can give you now, if you please, the rest produced by

Ptolemy and the other astronomers, or I can do so after you have spoken
what you have to say in answer to Aristotle.

salv.: The arguments brought up are of two kinds: some have respect
to the terrestrial accidents without any relation to the stars, and others are
taken from the phenomena and observations of things celestial. The argu-
ments of Aristotle are for the most part taken from things near at hand,
and he leaves the rest to astronomers; and therefore it is best to examine
these taken from experiments touching the Earth and then proceed to
those of the other kind. And as Ptolemy, Tycho,¹⁶ and the other philoso-
phers produced certain arguments besides those of Aristotle which they
adopted and confirmed, we will combine them, so we will not answer
twice to the same objections. Therefore, Simplicius, choose whether you
will recite them yourself or have me ease you of this task, for I am ready to
serve you.

simp.: It is better that you quote them, because, as having taken more
pains in the study of them, you can produce them quicker and in greater
number.

saly: For the strongest reason, all allege that heavy bodies falling down-
wards move by a straight line perpendicular to the surface of the Earth, an
argument which is held to prove undeniably that the Earth is immovable.
For otherwise a tower from the top of which a stone is let fall, being car-
ried along by the rotation of the Earth in the time that the stone spends in
falling, would be transported many hundred yards eastwards, and so far
distant from the tower's foot would the stone come to the ground. This
effect they back with another experiment: letting a bullet of lead fall from
the round top of a ship that lies at anchor and observing the mark it makes
where it lights. This they find to be near the foot of the mast, but, if the
same bullet be let fall from the same place when the ship is under sail, it
will light as far from the former place as the ship has sailed in the time of
the lead's descent, because the natural motion of the ball being at liberty is
by a straight line towards the centre of the Earth.¹⁷

They fortify this argument with the experiment of a projectile shot on
high at a very great distance. For example, a ball shot out of a cannon
erected perpendicular to the horizon spends so much time in ascending
and failing that, in our parallel, both we and the cannon should be carried
by the Earth many miles towards the East, so that that the ball in its return

could never come near the piece but would fall as far west as the Earth had
run east. They again add a third and very evident experiment,¹⁸ namely,
that shooting a ball point-blank (or, as gunners say, neither above nor
under metal)¹⁹ out of a cannon towards the east, and afterwards another
with the same charge and at the same elevation or disport towards the
west, the range towards the west should be very much greater than the
other towards the east, for the ball that goes westward, while the gun is
carried along by the Earth eastward, will fall from the piece as far distant
as the aggregate of the two motions. On the contrary, from the range of
the ball shot eastward you are to subtract the space the piece moved, being
carried after it. Now suppose, for example, that the range of the ball shot
west was five miles and that the Earth in the same parallel and in the time
of the ball's ranging should move eastward three miles: the ball in this case
would fall eight miles distant from the cannon. But the range of the shot
towards the east would be but two miles long, for so much is the remain-
der after you have subtracted from the five miles of the range the three
miles the piece had moved towards the same part. But experience shows
the ranges to be equal; therefore, the culverin and consequently the Earth
are immovable. The stability of the Earth is confirmed again by two other
shots made north and south; ranges would always be wide, or towards the
west, as the target would be carried along with the Earth towards the cast,
whilst the ball is flying. Not only shots made by the meridians but also

those aimed east and west would prove uncertain; for those aimed east
would be too high, and those directed west too low, although they were
shot point-blank, as I said. The last shot point-blank describes a tangent
to the Earth, and the diurnal motion of the Earth continually descending
in the east and rising in the west would cause the ball to hit too high in the
east and too low in the west. But experience tells us the contrary, hence it
is requisite to say that the Earth is immovable.

simp.: Now, these in truth are solid reasons and such as I believe no
man can answer.

salv.: Perhaps they are new to you?

simp.: Really they are; and now I see with how many admirable exper-
iments Nature is pleased to favour us with which to assist us in the knowl-
edge of the truth. Oh! how exactly one truth agrees with another, and all
conspire to render each other inexpugnable!

sagr.: What say you, Simplicius? Do you think that Salviatus does
master and know how to unfold the Ptolemean and Aristotelian argu-
ments? Or do you think that any Peripatetic is equally versed in the Co-
pernican demonstrations?

simp.: Were it not for the high esteem that the past discourses have
begot in me for the learning of Salviatus, and for the acuteness of Sagre-
dus, I would by their good leave have gone my way without staying for
their answers, as it seems to me an impossible thing that such palpable
experiments should be contradicted. I would, indeed, without hearing
them further, confirm myself in my old persuasion, for, were it even wrong,
its being upheld by so many probable reasons would render it excusable.
And if these are fallacies, what true demonstrations were ever so fair?

sagr.: Yet it is good that we should now hear the responsions of Salvia-
tus; these, if they be true, must of necessity be infinitely more fair, and
those others must be misshapen, if the metaphysical axiom holds that true
and fair are one and the same thing, as also false and ugly. Therefore,
Salviatus, let us no longer lose time.

salv.: The first argument alleged by Simplicius, if I well remember,
was this. The Earth cannot move circularly, because such motion would
be violent and therefore not perpetual. And we call it violent because,
were it natural, its parts would likewise naturally move round, which is
impossible, for we see that it is natural for its parts to move with a straight
motion downwards. To this my reply is that I strongly wish Aristotle had
more clearly expressed himself when he said that its parts would likewise
move circularly. Moving circularly can be understood two ways: one is
that every particle or atom separated from its whole would move circular-
ly about its particular centre, describing its small circlets; the other is that,
as the whole globe moves about its centre in twenty-four hours, the parts
also will turn about the same centre in the same time. The first would be
no less an impertinency than if one should say that every part of the cir-
cumference of a circle ought to be a circle, or every part of the Earth ought
to be a globe, because of the axiom that the same reason is in the whole
and in the parts. But if he took it in the sense that the parts in imitation of
the whole move naturally round the centre of the whole globe in twenty-

four hours, I say that they do so; and it concerns you, in the place of
Aristotle, to prove that they do not.

simp.: This is proved by Aristotle in the same text, when he says that
the natural motion of the parts is the straight motion downwards to the
centre of the Universe, so that the circular motion cannot naturally be-
long to them.

salv.: But do you not see that these very words carry in them a refuta-
tion of this solution?

simp.: How? And where?

salv.: Does he not say that the circular motion of the Earth would be
violent and therefore not eternal? And that this is absurd, because the
order of the World is eternal?

simp.: He says so.

saly.: But if that which is violent cannot be eternal, then, by conver-
sion, that which cannot be eternal cannot be natural. But the motion of
the Earth downwards cannot in any way be eternal; therefore much less
can it be natural, nor can any other motion be natural to it, save only that
which is eternal. But, if we make the Earth move with a circular motion,
this may be eternal to it, and to its parts, and therefore natural.

simp.: The straight motion is most natural to the parts of the Earth
and is eternal to them, nor shall it ever happen that they do not move with
a straight motion, provided that impediments be removed.²⁰

salv.: You equivocate, Simplicius; and I will try to clear you up. Tell
me, do you think that a ship which should sail from the Straits of Gibral-
tar towards Palestine can eternally move towards the coast on an equal
course?

simp.: No, doubtless.

salv.: And why not?

SlMP.: Because that voyage is bounded and terminated between the
Pillars of Hercules and the shore of the Holy Land; and, the distance be-
ing limited, it is passed in a finite time, unless one by returning back
should begin the same voyage anew; this would be interrupted and not
continued motion.

salv.: Very true. But take the navigation from the Straits of Magellan
by the Pacific Ocean, the Moluccas, the Cape of Good Hope and from

thence by the same Straits, and then again by the Pacific Ocean, etc.; do
you believe that that may go on forever?

simp.: It may, for it is a circumgyration.

salv.: A ship then may in this voyage continue sailing eternally?

simp.: It may, in case the ship were incorruptible, but, as the ship de-
cays, the navigation must of necessity come to an end.

salv.: But in the Mediterranean, though the vessel were incorruptible,
yet she could not sail perpetually towards Palestine, that voyage being
limited. Two things then are required if a body must without intermission
move perpetually. One is that the motion of its own nature be interminate
and infinite; the other that the body be likewise incorruptible and eternal.

simp.: All this is necessary.

salv.: Therefore you may see how of your own accord you have con-
fessed it impossible that any body should move eternally in a straight line,
since straight motion, whether it be upwards or downwards, is bounded,
as you confessed, by the circumference and centre. Therefore, as straight
motion is not of its own nature eternal, but most terminate, it cannot
naturally suit the Earth. Nay, as was said yesterday, Aristotle himself is
constrained to make the terrestrial globe eternally immovable. When again
you say that the parts of the Earth evermore move downwards, all imped-
iments being removed, you equivocate most egregiously. Far to the con-
trary, they must be contrasted and forced, if you would have them move;
for, when they have once reached the ground, they must be violently thrown
upwards that they may fall a second time. As to the impediments, these
only hinder their arrival at the centre; but if there were a well that did pass
through and beyond the centre, yet a clod of card? would not pass it,
unless, transported by its impetus, it should pass the same to return thith-
er again, and in the end there to rest. We ought to agree, therefore, that
the motion by a straight line does not belong naturally either to the Earth
or to any other body while the Universe retains its perfect order. And,
unless you will grant them the circular motions, your best way will be to
defend and maintain their immobility.

simp.: As to their immovableness I still rely on the arguments of Aristotle,
and moreover on those alleged by yourself, and I conceive it will be hard to
refute them.

saly.: We come therefore to the second argument, which was that
those bodies of which we are sure that they move circularly have more
than one motion, unless it be the Primum Mobile. Therefore, if the Earth
did move circularly, it ought to have two motions; from which would
follow a change in the rising and setting points of the fixed stars, but as
this effect is not perceived to ensue, therefore, etc. The most proper and
genuine answer to this allegation is contained in the argument itself; even
Aristotle puts it in our mouths, and it is impossible, Simplicius, that you
should not have seen it.

SlMP.: I neither have seen it nor do I yet see it.

saly.: This cannot be; surely the thing is very plain.

simp.: I will, with your leave, cast an eye upon the text.

sagr.: We will have the text brought forthwith.

slmp.: I always carry it about me. See, here it is, and I know the place
perfectly well, which is in Book ii of De coelo, chapter 16. Here it is, Text
97:²¹ "Furthermore all that are carried with circular motion seem to stay
behind, and to move with more than one motion, except the first sphere;
wherefore it is necessary that the Earth move with two motions, whether
it be carried about the centre, or placed in the centre. But if it be so, there
would of necessity be alterations and conversions made amongst the fixed
stars. But no such thing is seen to be done, and the same star always rises
and sets in the same place." In all this I do not find any fallacy, and I think
the argument is very forcible.

saly.: This new reading of the place has confirmed me in the fallacy of
the syllogism and moreover discovered another falsity. Therefore, look.
The positions, or, if you will, conclusions, which Aristotle endeavours to
oppose are two. One is that of those who, placing the Earth in the midst
of the World, make it move in itself about its own centre. The other is of
those who, constituting it far from the centre, make it revolve with a cir-
cular motion about the centre of the Universe. And both these positions
he conjointly impugns with one and the same argument. Now I affirm
that he is out in both the one and the other point and that his error against
the first position is an equivocation or paralogism, and his mistake touch-
ing the second is a false consequence. Let us begin with the first assertion,
which constitutes the Earth in the the midst of the World, and makes it
move on itself about its own centre; and let us confront it with that objec-

tion of Aristotle which says: All movables that move circularly seem to
stay behind and move with more than one motion, except the first sphere
(i.e., the Primum Mobile). Therefore the Earth moving about its own cen-
tre, being placed in the middle, ought of necessity to be carried about in
two ways and also to stay behind. But, if this were so, it would follow that
there should be a variation in the rising and setting of the fixed stars, as we
do not perceive this to be done; therefore, the Earth does not move, etc.

Here is the paralogism, and to discover it I will argue with Aristotle in
this manner.²² Thou sayest, O Aristotle, that the Earth placed in the mid-
dle of the World cannot move in itself (i.e., upon its own axis), for then we
would have to allow it two motions; therefore, if we did not have to allow
it more than one motion, thou wouldst not hold it impossible for it to
move only with that one; for thou wouldst unnecessarily have confined
the impossibility to the plurality of motions, if, in case it had no more but
one, yet it could not move with that. Now, of all the bodies in the World,
thou makest but one alone to move with one sole motion, and all the rest
with more than one; and this same body thou affirmest to be the first
sphere, namely, that by which all the fixed and erratic stars seem harmoni-
ously to move from east to west; hence, in case the Earth could be that
first sphere, that by moving with one motion only may make the stars
appear to move from east to west, thou wouldst not deny it that motion.
But he that affirms that the Earth, being placed in the midst of the World,
moves about its own axis, ascribes unto it no other motion save that by
which all the stars appear to move from east to west; and so it comes
actually to be the first sphere, which thou thyself acknowledgest to move
with but one motion only. It is therefore necessary, O Aristotle, if thou
wilt conclude anything, that thou demonstrate that the Earth, being placed
in the midst of the World, cannot move with so much as one motion only;
or, else, that not even the first sphere can have one sole motion; for other-
wise thou dost in thy very syllogism both commit the fallacy and manifest
it, denying and at that very time proving the same thing.

I come now to the second position, namely, of those who, placing the
Earth far from the centre of the Universe, make it movable about the
same; that is, make it a planet and erratic star. Against these the argument
is directed, and as to form is concludent, but fails in matter. For it being
granted that the Earth does move in that manner, and with two motions,

yet it does not necessarily follow that, though it were so, it should make
alterations in the risings and settings of the fixed stars, as I shall in its
proper place explain. And here, I could gladly excuse Aristotle; rather I
could highly applaud him for having lit upon the most subtle argument
that could be produced against the Copernican hypothesis; and if the ob-
jection be ingenious and to outward appearance most powerful, you may
see how much more acute and ingenious the solution must be and not to
be found by a mind less piercing than that of Copernicus. Again, from the
difficulty in understanding it you may argue the so much greater difficul-
ty in finding it. But for the present let us suspend our answer, which you
shall understand in due time and place, after we have repeated the objec-
tion of Aristotle, and much strengthened it in his favour at that.

Now we pass to Aristotle's third argument, touching which we need
give no further reply, as it has been sufficiently answered betwixt the dis-
courses of yesterday and today: inasmuch as he argues that the motion of
heavy bodies takes place naturally by a straight line to the centre and then
enquires whether to the centre of the Earth or to that of the Universe and
concludes that they tend naturally to the centre of the Universe but acci-
dentally to that of the Earth.

Therefore we may proceed to the fourth, upon which it is requisite
that we stay some time, for the reason that it is founded upon that exper-
iment from which the greater part of the remaining arguments derive all
their strength. Aristotle says that it is a most convincing argument of the
Earth's immobility to see that projectiles thrown or shot upright return
perpendicularly by the same line unto the same place from whence they
were shot or thrown. And this holds true, although the motion be of a
very great height. So that hither may be referred the argument taken from
a shot fired directly upwards from a cannon, as also that other used by
Aristotle and Ptolemy, of the heavy bodies that, falling from on high, are
observed to descend by a direct and perpendicular line to the surface of
the Earth. Now, that I may begin to untie these knots, I demand this of
Simplicius: in case one should deny to Ptolemy and Aristotle that weights
in falling freely from on high descend by a right and perpendicular line,
that is, directly to the centre, what, means would he use to prove it?

simp.: The means of the senses, which assure us that that tower or
other altitude is upright and perpendicular, and shew us that that stone
slides along the wall, without inclining a hair's breadth to one side or
another, and lights on the ground just under the place from where it was
let fall.

salv.: But if it should happen that the terrestrial globe did move round,
and consequently carry the tower also along with it, and that the stone did
then also graze and slide along the side of the tower, what must its motion
be then?

simp.: In this case we may rather say its motions, for it would have one
wherewith to descend from the top to the bottom and should then have
another to follow the course of the said tower.

salv.: So that its motion should be compounded of two; from this it
would follow that the stone would no longer describe that simple straight
and perpendicular line but one transverse and perhaps not straight.

SlMP.: I can say nothing of its nonrectitude, but this I know very well:
that it would of necessity be transverse.

saly: You see then that, merely observing the falling stone to glide
along the tower, you cannot certainly affirm that it describes a line which
is straight and perpendicular unless you first suppose that the Earth stands
still.

simp.: True; for, if the Earth should move, the stone's motion would be
transverse and not perpendicular.

salv: Then will you please hold the paralogism of Aristotle and Ptolemy
to be evident and manifest, and discovered by yourself, for in it that is sup-
posed for known which is intended to be demonstrated.

simp.: How can that be? To me it appears that the syllogism is rightly
demonstrated without petition of principle.

salv: You shall see how it is; first answer me, does he not lay down the
conclusion as unknown?²³

simp.: Unknown: why, otherwise, demonstrating it would be super-
fluous.

salv: But the middle term, ought not that to be known?

simp.: It is necessary that it should; for otherwise it would be proving
ignotum per aeque ignotum, the unknown by the equally unknown.

salv: Our conclusion which is to be proved, and which is unknown, is
it not the stability of the Earth?

simp.: It is.

salv: The middle term, which ought to be known, is it not the straight
and perpendicular descent of the stone?

simp.: It is so.

salv: But was it not just now concluded that we can have no certain
knowledge whether it shall be direct and perpendicular unless we first
know that the Earth stands still? Therefore in your syllogism the certainty
of the middle term is assumed from the uncertainty of the conclusion.
You may see, then, what and how great the paralogism is.

sagr.: I would defend Aristotle in favour of Simplicius. Should the
tower move, it would be impossible that the stone should fall gliding along
the side of it; and therefore from its falling in that manner the stability of
the Earth is inferred.

simp.: It is so; for if you would have the stone in descending to graze
along the tower, while being carried around by the Earth, you must allow
the stone two natural motions, to wit, the straight motion towards the
centre and the circular motion about the centre, which is impossible.

salv: Aristotle's defense then consists in the impossibility, or at least in
his esteeming it an impossibility, that the stone should move with a mo-
tion mixed of right and circular. For, if he did not hold it impossible that
the stone could move at once to the centre and about the centre, he would
have understood that it might come to pass that the falling stone might in
its descent graze the tower as well when it moved as when it stood still.
Consequently, he ought to have perceived that from this grazing nothing
could be inferred touching the mobility or immobility of the Earth. But
this does not any way excuse Aristotle; because he ought to have expressed
it, if he had had such a notion, it being so material a part of his argument.
Also because it cannot be said that such an effect is impossible or that
Aristotle did esteem it so. The first cannot be affimed, for by and by I shall
shew that it is not only possible but necessary; nor can the second be
averred, for Aristotle himself grants that fire moves naturally upwards in a
right line, and moves about with diurnal motion, imparted by the heavens
to the whole element of fire and the greater part of the upper air. If there-
fore he held it possible to mix the straight motion upwards with the circu-
lar communicated to the fire and air from the concave of the sphere of the
Moon, much less ought he to account impossible the mixture of the straight

motion of the stone downwards with the circular which we presuppose
natural to the whole terrestrial globe, of which the stone is a part.

simp.: I see no such thing; for, if the element of fire revolves round
together with the air, it is a very easy, even a necessary, thing that a spark of
fire which mounts upwards from the Earth, in passing through the mov-
ing air, should receive the same motion, being a body so thin, light, and
easy to be moved. But that a very heavy stone, or a cannon ball, that
descends from on high, and that is at liberty to move whither it will,
should suffer itself to be transported either by the air of any other thing is
altogether incredible. Besides that, we have the experiment which is so
proper to our purpose, of the stone let fall from the round top of the mast
of a ship, which, when the ship lies still, falls at the foot of the mast, but,
when the ship moves, falls as far distant from that place, as how far the
ship in the time of the stone falling had run forward.

salv.: There is a great disparity between the case of the ship and that of
the Earth, if the terrestrial globe be supposed to have a diurnal motion.
For it is manifest that, as the motion of the ship is not natural to it, the
motion of all those things that are in it is accidental, whence it is no won-
der that the stone which was retained in the round top, being left at liber-
ty, descends downwards without any obligation to follow the motion of
the ship.²⁴ But the diurnal conversion is ascribed to the terrestrial globe
for its proper and natural motion, and, consequently, it is so to all parts of
the said globe; and, being impressed by Nature, is indelible in them. There-
fore that stone that is on the top of the tower has an intrinsic inclination
to revolve about the centre of its whole in twenty-four hours, and it exer-
cises this same natural instinct eternally, be it placed in any state whatso--
ever. To be assured of the truth of this, you have to do no more than alter
an antiquated impression made in your mind, and to say: as I, hitherto,
held it to be the property of the terrestrial globe to rest immovable about
its centre, and never doubted or questioned but that all particles do also
naturally remain in the same state of rest; it is reason, in case the terrestrial
globe did move round by natural instinct in twenty-four hours, that the
intrinsic and natural inclination of all its parts should also be to follow the
same revolution. And thus, without running into any inconvenience, one
may conclude that, since the motion conferred by the force of oars on the
ship,²⁵ and by it on all the things that are contained within her, is not

natural but foreign, it is very reasonable that that stone, being separated
from the ship, should bring itself back to its natural disposure and return
to exercise its pure simple instinct give it by Nature. To this I add that it is
necessary that at least that part of the air which is beneath the greater
heights of mountains should be transported and carried round by the rough-
ness of the Earth's surface; or that, as being mixed with many vapours and
terrene exhalations, it should naturally follow the diurnal motion, which
does not occur in the air about the ship rowed by oars. Therefore your
arguing from the ship to the tower has not the force of a conclusion,
because the stone which falls from the round top of the mast enters into a
medium which is unconcerned in the motion of the ship; but that which
departs from the top of the tower finds a medium that has a motion in
common with the whole terrestrial globe, so that, rather than being hin-
dered, it is assisted by the motion of the air and may follow the universal
course of the Earth.

simp.: I cannot conceive that the air can imprint in a very great stone,
or in a heavy globe of wood or ball of lead, as suppose of two hundred-
weight, the motion wherewith itself is moved, and which it perhaps com-
municates to feathers, snow, and other very light things. On the contrary,
I see that a weight of that nature, being exposed to the most impetuous
wind, is not thereby removed an inch from its place; now consider with
yourself whether the air will carry it along therewith.

salv.: There is a great difference between your experiment and our
case. You introduce the wind blowing against that stone, supposedly in a
state of rest, and we expose the stone to the air which already moves, with
the same velocity as the stone, so that the air is not to confer a new motion
upon it but only to maintain or, to say better, not to hinder the motion
already acquired. You would drive the stone with a strange and preternat-
ural motion, and we desire to conserve it in its natural one. If you would
produce a more pertinent experiment, you should say that it is observed,
if not with the eye of the forehead, yet with that of the mind, what would
happen if an eagle that is carried by the course of the wind should let a
stone fall from its talons. Now as the stone went along with the wind
when it was let go, and after it started on its fall entered into a medium
that moved with equal velocity, I am very confident that it would not be

seen to descend in its fall perpendicularly but would move with a trans-
verse motion.

simp.: But it should first be known how such an experiment may be
made; and then one might judge according to the event. In the meantime
the effect of the ship hitherto inclines to favour our opinion.

salv.: Well you said "hitherto," for perhaps it may change counte-
nance anon. And that I may no longer hold you in suspense, tell me,
Simplicius, do you really believe that the experiment of the ship squares so
very well with our purpose that it ought to be believed that that which we
see happen in it ought also to take place in the terrestrial globe?

simp.: As yet I am of that opinion; and, though you have alleged some
small disparities, I do not think them of so great a moment that they
should make me change my judgment.

salv.: I rather desire that you would continue in that belief and hold
for certain that the effect of the Earth would exactly answer that of the
ship, provided that when it shall appear prejudicial to your cause, you will
not be of humour to alter your thoughts. You say that, when the ship
stands still, the stone falls at the foot of the mast and that, when she is
under sail, it lights far from thence; therefore, by conversion, from the
stone's falling at the foot is inferred the ship's standing still, and from its
falling far from thence is inferred her moving. And because that which
occurs to the ship ought likewise to befall the Earth, therefore from the
falling of the stone at the foot of the tower is necessarily inferred the im-
mobility of the terrestrial globe. Is this not your argumentation?

simp.: It is; and reduced into such conciseness that it is become most
easy to be apprehended.

salv.: Now tell me; if the stone let fall from the round top when the
ship is in swift course, should fall exactly in the same place of the ship in
which it falls when the ship is at anchor, what service would these experi-
ments do you, to the end of ascertaining whether the vessel does stand still
or move?

simp.: Exactly none. As, for example, from the beating of the pulse
one cannot know whether a person is asleep or awake, seeing that the
pulse beats in the same manner in sleeping as in waking.

salv.: Very well. Have you ever tried the experiment of the ship?

simp.: I have not; but yet I believe that those authors who allege the
same have accurately observed it; besides that, the cause of the disparity is
so manifestly known that it admits of no questions.

salv.: It may be that those authors believed in it, without having made
trial of it, and you yourself are a good witness to the point. For you, with-
out having examined it, allege it as certain and in good faith rely on their
authority; as it is now not only possible but obvious that they also relied
on their predecessors, without ever arriving at one that had made the ex-
periment. For, mark you, whosoever shall perform it shall find the event
succeed quite contrary to what has been written of it. That is, he shall see
the stone fall at all times in the same place of the ship, whether it stand still
or move with any velocity whatsoever. So that, the same holding true in
the Earth as in the ship, one cannot, from the stone's falling perpendicu-
larlv at the foot of the tower, conclude anything touching the motion or

rest of the Earth.

simp.: If you were referring me to any other means than to direct expe-
rience, I verily believe our disputations would not come to an end in haste;
for this seems to me a thing so remote from all human reason that it leaves
not the least place for credulity or probability.

saly.: And yet it has left place in me for both.

simp.: How is this? You have not made a hundred tests, no, not even
one test, and you so confidently affirm it for true? I for my part will return
to my incredulity and to the confidence I had that the experiment has
been tried by the principal authors who made use of it and that the event
turned out as they affirm.

salv.: I am assured that the effect will ensue as I tell you without ex-
periment, for so it is necessary that it should, and I further add that you
yourself know that it cannot fall out otherwise, however you feign or seem
to feign that you know it not.²⁶ . . .

notes

¹      GF integranti.

²      The controversy begins here about the merits of the two systems, each taken
as a whole. It should again be made clear (see our Introduction) that the oppo-
sition is not between two astronomical theories but between two contrasting
views about the Universe, the place of man, and the role of science. The Athe-

nian School, beginning with Socrates and Plato, had defined the goal of knowl-
edge in terms of a hierarchy of ideas leading up to the idea of the Good. Its
physical system, even in Aristotle the "naturalist," was only a reflection of that
hierarchy. The things of heaven are symbols of immutability and perfection,
as such nearer to the gods, and metaphysically different from the world under
the Moon. Astronomy is in the Platonic view a kind of "astro-theology" and
linked with the science of the soul more than with that of matter. The math-
ematician is told he has nothing to do with the physicist. The idea seemed
established for all further speculation, even including the Stoics, who other-
wise tended to unify the universe under a single "physical" law. "Cosmology,"
says Posidonius, "is made up of two parts: one which deals with stars and
planets, with the apparent magnitude of the Sun and Moon, with their revo-
lutions and suchlike problems which concern also mathematicians; the other
part, which is reserved for physicists, deals with the problems of the essence of
the Universe, with the matter and form of the Sun and the stars, with the
question whether the Universe was once born or not, whether it is animated
or not, perishable or not, ruled by a Providence or not." This is the point of
view of humanism, of the Church, and of the docta pietas which strives to
unite both. Galileo is contending with a formidable array of allied ideas, which
all claimed the protection of religious orthodoxy. He has therefore to pick his
way amid heated plowshares.

³ De gener. anim. v. 2. 781 a 20.

⁴ De coelo ii. 14. 296 b 3. The motion is revolution around the Sun. The "An-
cient" is Philolaus. This subject is taken up again in the Third Day.

⁵      The concept of relativity of motion was already stated by Copernicus. De
revol. \. 5: "inter mota aequaliter ad eadem non percipitur motus, inter visum
dico et videns." Galileo went further and distinguished between kinematic
and dynamic relativity, as shown by his theory of tidal motion developed in
the Fourth Day of this Dialogue. But, although he believed that tangential
acceleration can be physically detected, he still tried to prove the Copernican
principle of "optical" relativity above as extended to both circular and straight
uniform motion in the case of the Earth.

⁶     Cf. Phys. iv; De motu anim. 698 b 8. The "good school" is that of Democritus.
Galileo rightly surmises that Aristotle must have misunderstood the physical
doctrine of the Atomists. Cf. E Enriques and G. de Santillana, Histoire de la
Pensee scientifique, III, 341; V, 48 ff. As we have said in the previous note,
Copernicus states only the relativity of motion, defined in optical terms.

⁷      "To a nail's thickness." The origin of this expression of classical rigor is artistic
and is to be found in a saying by Polycletus the sculptor. Cf. Diels, Fmgmente
der Vorsokratiker, I, 28B.

⁸      As you see in a mill, wherein the implicated cogs set the wheels on moving (S).

⁹      This was the name Galileo gave to the satellites of Jupiter he had just discov-
ered (January 7, 1610), on the advice of his faithful friend, Belisario Vinta. It
was intended to honor the Grand Duke of Tuscany and to put his contested
discover}' under his protection. This was no small gamble on the part of both
men, and especially of Vinta, who was Secretary of State. For the satellites
were thus linked so publicly with the prestige of the House of Medici that it
became mandatory for them to exist.

¹⁰    The precession of the equinoxes, with a 26,000-year cycle.

¹¹     Aristotle was not explicit about the hardness of the spheres, but that certainly
was the logic of his position, as Callippus drew it in his own time and later
Ptolemy (not in the Almagest, which is purely geometrical, but in the Hypoth-
eses oil the Planets). There was interminable wrangling on the subject in Gali-
leo's time. Tycho had strongly opposed the crystal theory, and even Scheiner
in his Rosa Ursina came out against it, with many quotations from Scripture
and from lav authors.

¹²    The planets moving "the contrary way" because of their orbital motion; the
starry sphere insomuch as it follows the precession of the equinoxes.

¹³    Aristotle maintained that part of the upper elemental sphere must take part in
the rotation (cf. Meteor, i. 7. 344 a 11), because comets and shooting stars
belong thereto.

¹⁴ Note on the Primum Mobile. That the first mover is outside the world follows
for Aristotle (Phys. viii) from (a) the assumption that movement must origi-
nate either from the center or from the circumference, these being the only
"beginnings"; (b) the assumption that the movement directly imparted by the
first mover must be the fastest of all movements, since all impetus slows down
in transmission; (c) the "observed fact" that the twenty-four-hour rotation of
the fixed stars is the fastest of all heavenly movements. We thus get the view
that the prime mover is outside. And it must be itself unmoved. To all effects
it is a kind of metaphysical animal, incorporeal and unextended, twirling, as it
were, the sphere between its paws. To soften the rather harsh transition, Aris-
totle thought up the Primum Mobile, a kind of semimaterial transmission belt
from the thought of God to the sphere of fixed stars. But how can movement

be imparted to it, since this implies either pushing or pulling? The Metaphysics
attempts an answer (1072 a 20). The argument begins, reasonably enough,
with two choices: either we can explain the world by way of something which
is eternally moved with an unceasing regular cyclic motion, or "we shall have
to regard everything as coming from 'Night,' and 'all-things-together' and
'not-being.'" Since the second choice is out of the question, we have only the
first. "And since that which is moved while it moves is intermediate, there is
something which moves without being moved; something eternal which is
both substance and actuality. Now it moves in the following manner. The
object of desire and the object of thought move without being moved. The
primary objects of desire and thought are the same (i.e., the Good) .... That
the final cause may apply to immovable things is shown by the distinction of
its meanings. For the final cause is not only 'the good for something' but also

'the good which is the end of some action.' In the latter sense it applies to
immovable things, although in the former it does not; and it causes motion as
being an object of love, whereas all other things cause motion because they are
themselves in motion."

Such is the final answer, and it is not very satisfactory. But there is more in it
than meets the eye (possibly even Aristotle's). The Primum Mobile, as a totally
undifferentiated sphere with no point of reference outside, is in a state in
which motion cannot be told from rest; hence no physical force has to be
exerted to keep it moving. Its motion, in other words, is at last really a "state,"
with respect to whatever there is outside. The only fixed point of reference
which defines that motion is inside; it is the "immobile" Earth. Aristotle, in
his own peculiar way, has formulated a principle of relativity which rejoins
that of Galileo, who is suggesting that we might just as well call the Earth the
Primum Mobile.

15    This is a crucial point in the centuries-old discussion on the motion of the
Earth. The thinking evidenced here by Aristotle appeals so strongly to com-
mon sense that Tycho himself accepted it and that Galileo will have to spend
many weary pages in refuting it. Heath sees in the passage above, De coelo 296
a 34 ff, that we have quoted already in the First Day, the simple analogical
argument that the Earth, if it has circular motion, must have two motions
because all planets have them, and oblique to each other. The text, however, as
Cherniss has pointed out, says really that the planets have "more than one
apart from the primary (i.e., diurnal motion)." This would make three at least
and has been thus understood by Themistius in antiquity (De coelo, pp. 137,
29—32 [Landauer trans.]). Editors, even in our time, as is seen in Guthrie's

translation, have understood the jiXei.ovt imccl. QopaZ e§w if]L. nponrjL, of the
best codices (EJLMT) through the amplified text of F, e|o> t>;c jrpomjt aoaip-
a£, which is but a mistaken gloss, whereas H reads <popat which is also proba-
bly a gloss but a correct one. Thus it appears that Aristotle is combining a
sophistic analogy with an appeal to phenomena. What he is saying—and Ga-
lileo reads him between the lines rightly—is: While the (unnamed) Pythagore-
ans may claim to have accounted for the lag of the planets by their own idea of
a motion in the zodiac from west to east, and for the primary or diurnal revo-
lution by the rotation of the Earth, there are still other planetary motions
which on their principles they could explain only by giving the Earth a sec-
ond, alternately tilting, motion. (Some such motion may be meant in Plato's
famous passage Timacits 40B). But the addition of this motion would result in
"excursions and turnings" of the fixed stars; hence, etc. With these motions, it
is to be noted, the excursions in latitude of the planets would be explained,
but not their retrogressions. It is therefore conceivable that Aristotle had in
mind the other system of figurative motions suggested by Eudoxus in his ho-
mocentric theory, which did account for the retrogressions, by means of four
angular components. But there is no evidence, that he thought of them at this
point, although the}- are implied in another passage. Met. 1073 a 28. What
appears from all this is that the straight Pythagorean idea (rediscovered by
Copernicus) that two motions are enough to explain all the major phenomena
had been lost sight of in Aristotle's milieu and that his own unaided imagina-
tion led him and his successors into a battle with windmills.

¹⁶    The reasons given by Ptolemy against the movement of the Earth are to be
found in Book i of the Almagest, chaps. 4-7. Galileo discusses them only in
the Third Day and rather cursorily. As forTycho's reasons (Epistolac astronom-
icae [1596], pp. 167 and 188 ff), they can be summarized as follows: (1) a
weight falling; or thrown would land with a considerable western deflection;

(2) the sphere of the fixed stars would be removed to an immense distance,
seven hundred times as far as Saturn, and one cannot see the reason for all this
wasted and starless space (which seems to have been also in a way the objec-
tion of Archimedes to Aristarchus); (3) the apparent diameter of the stars that
Tycho takes for real would indicate in that case dimensions which are beyond
the credible; (4) the "third motion" of the Earth in Copernicus' theory, which
causes the Earth to "rotate" around itself in a year, looks like a most artificial
contrivance; (5) how is it possible that the axis and the center should have two
different motions in a single simple body, not to speak of a third? Later, Tycho
had growing doubts about his denial of the rotation of the Earth on its own
axis, as appears in the Progymnasmata, p. 662. But he never came over to the

affirmative position, and, in fact, he claimed to have shaken Rothmann's Co-
pernican faith.

¹⁷    Was the experiment actually performed by someone? We shall come again
upon this point on page 158. The whole trend of the argument implies here,
as so often with Galileo, a "thought experiment." But in his letter to Ingoli
(Opere, VI, 545) he says: "I have been twice as good a philosopher as they are,
for that they to saying what is contrary in effect have added the lie, saying they
knew it from experiment, and I have made the experiment; previous to which
natural reason had most firmly persuaded me that the effect had to come out
as it actually did." In 1640 Gassendi performed the experiment again (De
motu impressu in motore translate in Opusc. phil. [1658]). Before that time
Giovanni Cotugno, a lector in Bologna, had tried his hand at the problem by
causing arrows to be shot vertically, with what appeared, to him contradictory
results (Chiaramonti, Difesa, p. 338). But the earliest experiments had been
carried out by Bruno, who came to the correct conclusion. Galileo avoids
mentioning Bruno on an}' occasion, for obvious reasons of prudence, but he
must have read the following passage in La Cena dellc Ceneri ("Ash Wednes-
day"), III, 5: "Let there be two, one of them on the ship, which is running, the
other outside at rest, both of them having their hand in about the same point
of the air. and from that same spot at the same time let each drop a stone,
without giving it any push; that of the former, without losing ground or devi-
ating one line, will come down at the prefixed spot on the ship; and that of the
latter win be left behind."

¹⁸     This was an argument of Tycho's.

¹⁹     GF simply di volata, "point-blank."

²⁰    Aristotle might have said that the straight movement, being between contrar-
ies, is always a passage from the potential to the actual. The element of earth as
a whole is always getting actualized (so long as there is a stone falling some-
where), but, like the other elements, it retains, too, an eternal potentiality due
to their nature, as the closest to pure matter. If the elements were wholly actu-
alized, hence separated, the world process would cease to be. Actualization
will make earth ever more into the perfect sphere of earth, without ever get-
ting to that point and with each part only partaking in the accomplishment of
the whole, for stone and clod are accidental parts and not true forms; whereas
the heavens in their circular movement are eternally actualized, as each part of
them is too. In conclusion, Aristotle might accuse Salviati of being somewhat
sophistical—as, indeed, he has to be, having accepted his opponent's terrain
of materialized geometric lines.

²¹     This capital passage (given in Latin in the original) has been quoted and dis-
cussed already (see n. 14 above). Galileo reads, as his text indicates, "except
the first sphere" rather than "in addition to the first sphere," as modern critics
have suggested, but he understands it correctly.

²²    Galileo is parodying here the manner of scholastic disputants and their rhe-
torical devices taken from the law courts.

²³    The syllogism under discussion would be in its canonic form:

A. If the Earth rotates, the body cannot fall perpendicularly.
B. But the body does fall perpendicularly.
C. Therefore the Earth does not rotate.

²⁴     Salviati is here only taking up the Aristotelian cue and playing with his oppo-
nent. His own ideas about the experiment will come later.

²⁵    That you may not suspect my translation, or wonder what Oars have to do
with a ship, you are to know that the Author intends the Gallies used in the
Mediterrane (S).

²⁶    We have seen (n. 17) that Galileo may have actually tried the experiment,
were it only for purposes of demonstration. But what he is giving here is the
carefully worked-out "thought experiment." Professor Koyre has devised a subtle
illustration to show whv the Aristotelians were also willing to bet against Ga-

lileo and Bruno, sight unseen, for it is clear that neither side thought it worth
while actually to go through with the test. Imagine the boat passing under a
bridge, and that two stones are dropped almost contiguously, one from the
bridge, the other from the top of the mast. Why should the two stones behave
differently? To suppose the contrary would amount to attributing to them a
"memory" of where they started from which makes them different. A stone is
a stone is a stone; it has only one simple capacity, which is to go down. A
second instinct to horizontal motion instilled in it by outside causes would
have sounded, to a hardheaded Aristotelian, like anthropomorphic fantasy.
And, in fact, Antonio Rocco would not yield. "In the first place," he replied,
"I will go on denying that it so happens; and if the contrary were proved to me
by convincing experiment, I would look for other reasons why it might hap-
pen."

---