From Objection II (Mersenne): (Deception and Certainty)

It is not, however, necessary to suppose that God is a deceiver in order to explain your being deceived about matters which you think you clearly and distinctly know. The cause of this deception could lie in you, though you are wholly unaware of it. Why should it not be in your nature to be subject to constant - or at least very frequent - deception? How can you establish with certainty that you are not deceived, or capable of being deceived, in matters which you think you know clearly and distinctly? Have we not often seen people turn out to have been deceived in matters where they thought their knowledge was as clear as the sunlight? Your principle of clear and distinct knowledge thus requires a clear and distinct explanation, in such a way as to rule out the possibility that anyone of sound mind may be deceived on matters which he thinks he knows clearly and distinctly. Failing this, we do not see that any degree of certainty can possibly be within your reach or that of mankind in general.

Descartes’s Response:

But since I see that you are still stuck fast in the doubts which I put forward in the First Meditation, and which I thought I had very carefully removed in the succeeding Meditations, I shall now expound for a second time the basis on which it seems to me that all human certainty can be founded.

First of all, as soon as we think that we correctly perceive something, we are spontaneously convinced that it is true. Now if this conviction is so firm that it is impossible for us ever to have any reason for doubting what we are convinced of, then there are no further questions for us to ask: we have everything that we could reasonably want. What is it to us that someone may make out that the perception whose truth we are so firmly convinced of may appear false to God or an angel, so that it is, absolutely speaking, false? Why should this alleged 'absolute falsity' bother us, since we neither believe in it nor have even the smallest suspicion of it? For the supposition which we are making here is of a conviction so firm that it is quite incapable of being destroyed; and such a conviction is clearly the same as the most perfect certainty.

But it may be doubted whether any such certainty, or firm and immutable conviction, is in fact to be had.

It is clear that we do not have this kind of certainty in cases where our perception is even the slightest bit obscure or confused; for such obscurity, whatever its degree, is quite sufficient to make us have doubts in such cases. Again, we do not have the required kind of certainty with regard to matters which we perceive solely by means of the senses, however clear such perception may be. For we have often noted that error can be detected in the senses, as when someone with dropsy feels thirsty or when someone with jaundice sees snow as yellow; for when he sees it as yellow he sees it just as clearly and distinctly as we do when we see it as white. Accordingly, if there is any certainty to be had, the only remaining alternative is that it occurs in the clear perceptions of the intellect and nowhere else.

Now some of these perceptions are so transparently clear and at the same time so simple that we cannot ever think of them without believing them to be true. The fact that I exist so long as I am thinking, or that what is done cannot be undone, are examples of truths in respect of which we manifestly possess this kind of certainty. For we cannot doubt them unless we think of them; but we cannot think of them without at the same time believing they are true, as was supposed. Hence we cannot doubt them without at the same time believing they are true; that is, we can never doubt them.

It is no objection to this to say that we have often seen people 'turn out to have been deceived in matters where they thought their knowledge was as clear as the sunlight'. For we have never seen, indeed no one could possibly see, this happening to those who have relied solely on the intellect in their quest for clarity in their perceptions; we have seen it happen only to those who tried to derive such clarity from the senses or from some false preconceived opinion.

It is also no objection for someone to make out that such truths might appear false to God or to an angel. For the evident clarity of our perceptions does not allow us to listen to anyone who makes up this kind of story.

There are other truths which are perceived very clearly by our intellect so long as we attend to the arguments on which our knowledge of them depends; and we are therefore incapable of doubting them during this time. But we may forget the arguments in question and later remember simply the conclusions which were deduced from them. The question will now arise as to whether we possess the same firm and immutable conviction concerning these conclusions, when we simply recollect that they were previously deduced from quite evident principles (our ability to call them 'conclusions' presupposes such a recollection). My reply is that the required certainty is indeed possessed by those whose knowledge of God enables them to understand that the intellectual faculty which he gave them cannot but tend towards the truth; but the required certainty is not possessed by others. This point was explained so clearly at the end of the Fifth Meditation that it does not seem necessary to add anything further here.

From Objection VI (Mersenne): (Freedom of Belief)

The sixth difficulty arises in connection with the indifference that belongs to our judgement, or liberty. This indifference, you claim, does not belong to the perfection of the will but has to do merely with its imperfection; thus, according to you, indifference is removed whenever the mind clearly perceives what it should believe or do or refrain from doing. But do you not see that by adopting this position you are destroying God's freedom, since you are removing from his will the indifference as to whether he shall create this world rather than another world or no world at all? Yet it is an article of faith that God was from eternity indifferent as to whether he should create one world, or innumerable worlds, or none at all. But who doubts that God has always perceived with the clearest vision what he should do or refrain from doing? Thus, a very clear vision and perception of things does not remove indifference of choice; and if indifference cannot be a proper part of human freedom, neither will it find a place in divine freedom, since the essences of things are, like numbers, indivisible and immutable. Therefore indifference is involved in God's freedom of choice no less than it is in the case of human freedom of choice.

Descartes’s Response:

6. As for the freedom of the will, the way in which it exists in God is quite different from the way in which it exists in us. It is self-contradictory to suppose that the will of God was not indifferent from eternity with respect to everything which has happened or will ever happen; for it is impossible to imagine that anything is thought of in the divine intellect as good or true, or worthy of belief or action or omission, prior to the decision of the divine will to make it so. I am not speaking here of temporal priority: I mean that there is not even any priority of order, or nature, or of 'rationally determined reason' as they call it, such that God's idea of the good impelled him to choose one thing rather than another. For example, God did not will the creation of the world in time because he saw that it would be better this way than if he had created it from eternity; nor did he will that the three angles of a triangle should be equal to two right angles because he recognized that it could not be otherwise, and so on. On the contrary, it is because he willed to create the world in time that it is better this way than if he had created it from eternity; and it is because he willed that the three angles of a triangle should necessarily equal two right angles that this is true and cannot be otherwise; and so on in other cases. There is no problem in the fact that the merit of the saints may be said to be the cause of their obtaining eternal life; for it is not the cause of this reward in the sense that it determines God to will anything, but is merely the cause of an effect of which God willed from eternity that it should be the cause. Thus the supreme indifference to be found in God is the supreme indication of his omnipotence. But as for man, since he finds that the nature of all goodness and truth is already determined by God, and his will cannot tend towards anything else, it is evident that he will embrace what is good and true all the more willingly, and hence more freely, in proportion as he sees it more clearly. He is never indifferent except when he does not know which of the two alternatives is the better or truer, or at least when he does not see this clearly enough to rule out any possibility of doubt. Hence the indifference which belongs to human freedom is very different from that which belongs to divine freedom. The fact that the essences of things are said to be indivisible is not relevant here. For, firstly, no essence can belong univocally to both God and his creatures; and, secondly, indifference does not belong to the essence of human freedom, since not only are we free when ignorance of what is right makes us indifferent, but we are also free - indeed at our freest - when a clear perception impels us to pursue some object.

From Objection VI (Mersenne): (God’s power and necessary truths)

The eighth difficulty arises out of your reply to the Fifth Set of Objections. How can the truths of geometry or metaphysics, such as those you refer to, be immutable and eternal and yet not be independent of God? What sort of causal dependence on God do they have? Could he have brought it about that there has never been any such thing as the nature of a triangle? And how, may we ask, could he have made it untrue from eternity that twice four makes eight, or that a triangle has three angles? Either these truths depend solely on the intellect that is thinking of them, or on existing things, or else they are independent, since it seems that God could not have brought it about that any of these essences or truths were not as they were from all eternity

Descartes’s Response:

8. If anyone attends to the immeasurable greatness of God he will find it manifestly clear that there can be nothing whatsoever which does not depend on him. This applies not just to everything that subsists, but to all order, every law, and every reason for anything's being true or good. If this were not so, then, as noted a little earlier, God would not have been completely indifferent with respect to the creation of what he did in fact create. If some reason for something's being good had existed prior to his preordination, this would have determined God to prefer those things which it was best to do. But on the contrary, just because he resolved to prefer those things which are now to be done, for this very reason, in the words of Genesis, 'they are very good'; in other words, the reason for their goodness depends on the fact that he exercised his will to make them so. There is no need to ask what category of causality is applicable to the dependence of this goodness upon God, or to the dependence on him of other truths, both mathematical and metaphysical. For since the various kinds of cause were enumerated by thinkers who did not, perhaps, attend to this type of causality, it is hardly surprising that they gave no name to it. But in fact they did give it a name, for it can be called efficient causality, in the sense that a king may be called the efficient cause of a law, although the law itself is not a thing which has physical existence, but is merely what they call a 'moral entity'. Again, there is no need to ask how God could have brought it about from eternity that it was not true that twice four make eight, and so on; for I admit this is unintelligible to us. Yet on the other hand I do understand, quite correctly, that there cannot be any class of entity that does not depend on God; I also understand that it would have been easy for God to ordain certain things such that we men cannot understand the possibility of their being otherwise than they are. And therefore it would be irrational for us to doubt what we do understand correctly just because there is something which we do not understand and which, so far as we can see, there is no reason why we should understand. Hence we should not suppose that eternal truths 'depend on the human intellect or on other existing things'; they depend on God alone, who, as the supreme legislator, has ordained them from eternity.

From Objection VI (Mersenne): (Mind versus Senses)

Our ninth and most worrying difficulty is your assertion that we ought to mistrust the operations of the senses and that the reliability of the intellect is much greater than that of the senses. But how can the intellect enjoy any certainty unless it has previously derived it from the senses when they are working as they should? How can it correct a mistake made by one of the senses unless some other sense first corrects the mistake? Owing to refraction, a stick which is in fact straight appears bent in water. What corrects the error? The intellect? Not at all; it is the sense of touch. And the same sort of thing must be taken to occur in other cases. Hence if you have recourse to all your senses when they are in good working order, and they all give the same report, you will achieve the greatest certainty of which man is naturally capable. But you will often fail to achieve it if you trust the operations of the mind; for the mind often goes astray in just those areas where it had previously supposed doubt to be impossible.

Descartes’s Response:

9. If we are to get a clear view of what sort of certainty attaches to the senses, we must distinguish three grades of sensory response. The first is limited to the immediate stimulation of the bodily organs by external objects; this can consist in nothing but the motion of the particles of the organs, and any change of shape and position resulting from this motion. The second grade comprises all the immediate effects produced in the mind as a result of its being united with a bodily organ which is affected in this way. Such effects include the perceptions of pain, pleasure, thirst, hunger, colours, sound, taste, smell, heat, cold and the like, which arise from the union and as it were the intermingling of mind and body, as explained in the Sixth Meditation. The third grade includes all the judgements about things outside us which we have been accustomed to make from our earliest years - judgements which are occasioned by the movements of these bodily organs.

For example, when I see a stick, it should not be supposed that certain 'intentional forms' fly off the stick towards the eye, but simply that rays of light are reflected off the stick and set up certain movements in the optic nerve and, via the optic nerve, in the brain, as I have explained at some length in the Optics. This movement in the brain, which is common to us and the brutes, is the first grade of sensory response. This leads to the second grade, which extends to the mere perception of the colour and light reflected from the stick; it arises from the fact that the mind is so intimately conjoined with the body that it is affected by the movements which occur in it. Nothing more than this should be referred to the sensory faculty, if we wish to distinguish it carefully from the intellect. But suppose that, as a result of being affected by this sensation of colour, I judge that a stick, located outside me, is coloured; and suppose that on the basis of the extension of the colour and its boundaries together with its position in relation to the parts of the brain, I make a rational calculation about the size, shape and distance of the stick: although such reasoning is commonly assigned to the senses (which is why I have here referred it to the third grade of sensory response), it is clear that it depends solely on the intellect. I demonstrated in the Optics how size, distance and shape can be perceived by reasoning alone, which works out any one feature from the other features. The only difference is that when we now make a judgement for the first time because of some new observation, then we attribute it to the intellect; but when from our earliest years we have made judgements, or even rational inferences, about the things which affect our senses, then, even though these judgements were made in exactly the same way as those we make now, we refer them to the senses. The reason for this is that we make the calculation and judgement at great speed because of habit, or rather we remember the judgements we have long made about similar objects; and so we do not distinguish these operations from simple sense-perception.

It is clear from this that when we say 'The reliability of the intellect is much greater than that of the senses', this means merely that when we are grown up the judgements which we make as a result of various new observations are more reliable than those which we formed without any reflection in our early childhood; and this is undoubtedly true. It is clear that we are not here dealing with the first and second grades of sensory response, because no falsity can occur in them. Hence when people say that a stick in water 'appears bent because of refraction', this is the same as saying that it appears to us in a way which would lead a child to judge that it was bent - and which may even lead us to make the same judgement, following the preconceived opinions which we have become accustomed to accept from our earliest years. But I cannot grant my critics' further comment that this error is corrected 'not by the intellect but by the sense of touch'. As a result of touching it, we may judge that the stick is straight, and the kind of judgement involved may be the kind we have been accustomed to make since childhood, and which is therefore referred to as the 'sense' of touch. But the sense alone does not suffice to correct the visual error: in addition we need to have some degree of reason which tells us that in this case we should believe the judgement based on touch rather than that elicited by vision. And since we did not have this power of reasoning in our infancy, it must be attributed not to the senses but to the intellect. Thus even in the very example my critics produce, it is the intellect alone which corrects the error of the senses; and it is not possible to produce any case in which error results from our trusting the operation of the mind more than the senses.