Hume on Force and Vivacity and the Content of Ideas
Saint Anselm College
The everlasting universe of things
Flows through the mind, and rolls in rapid waves,
Now dark -- now glittering -- now reflecting gloom --
Now lending splendour, where from secret springs
The source of human thought its tributes brings
The secret strength of things
Which governs thought, and to the infinite dome
Of heaven is as a law, inhabits thee!
P. B. Shelley, Mont Blanc
I would willingly establish it as a general maxim in
the science of human nature, that when any impression
becomes present to us, it not only transports the mind to
such ideas as are related to it, but likewise communicates
to them a share of its force and vivacity.
David Hume, A Treatise of Human Nature
Jerry Fodor has recently suggested that Hume’s minimalist set of metaphysical assumptions about the causal machinery behind the processes we observe in cognition make him a useful philosopher to consider to see the basic value of a Representational Theory of Mind versus a Pragmatic theory, of seeing concepts as objects of mental contemplation, rather than as dispositions to act and classify objects. Looking a bit more closely at the role that force and vivacity play in Hume’s theory of abstract ideas reveals, however, that Hume may not be the ideal philosopher to make this case. The real content of thought for Hume lies in the customs and habits associated with ideas, and is felt through the force and vivacity associated with them.
All of us are familiar with Hume’s distinction between impressions and ideas: ideas are paler copies of impressions, differing only in force and vivacity from the original impressions from which they are derived. An idea and its corresponding impression do not differ in content but only in this force or vivacity.  This force and liveliness associated with each impression and idea is transmitted to the impression from the object in its formation and to each idea from the impression or idea from which it originates. In the physics of ideas that the Treatise attempts to establish, the force and vivacity is like a type of force, momentum, or impetus which can be transferred and is source of the agency of each idea as it acts and interacts in the mind. (I will use the word impetus to describe this force and liveliness of an idea or impression.)
The impetus of
ideas also accounts for the nature of belief, according to Hume:
Belief is a habit or custom associated with an idea, and this habit is manifested in the force and vivacity with which the idea is held.
Thus it appears, that the belief or assent, which always attends the memory and senses, is nothing but the vivacity of those perceptions they present; and that this alone distinguishes them from the imagination. To believe is in this case to feel an immediate impression of the senses, or a repetition of that impression in the memory. 'Tis merely the force and liveliness of the perception, which constitutes the first act of the judgment, and lays the foundation of that reasoning, which we build upon it, when we trace the relation of cause and effect. (Treatise I, I, 1)
Again, Hume takes pains to stress that an idea believed only differs from one entertained by its impetus, or force. While the impetus is distinct from the content, it also is felt and felt in a very determinate way: “An idea assented to feels different from a fictitious idea, that the fancy alone presents to us: And this different feeling I endeavour to explain by calling it a superior force, or vivacity, or solidity, or firmness, or steadiness.” (Treatise I, I, 7)
This all seems clear enough, but it should give us pause. Hume has attempted to separate the content of an idea from its causal efficacy in the flow of ideas and actions that involve it. Yet he cannot help but recognize that we are aware of this force or impetus; it is part of the phenomenology of having an idea, although not its content. (If that even makes sense.) This puzzle becomes central to Hume’s theory of cognition, once one sees the role that custom, habit, and their associated felt forces play in Hume’s account of general thought.
All ideas are particular and determinate for Hume. Ideas become general by bringing to mind an indefinite number of other ideas according to a custom or habit associated with the word or idea. The meaning of a general idea, then, is the custom or habit by which it comes indifferently to bring to mind a wide range of associated ideas. This custom and the impetus associated with a general idea are mysterious things to be known by analogy to other cases.  Just as we feel the efficacy of the habit of a belief to make us assent and act upon the content of an idea, we feel the efficacy of the custom or habit by which a particular idea can bring up an entire set of other particular ideas and act generally. And this impetus is the only felt difference between a general idea and the corresponding particular idea (just as there is no difference between the contents of a believed and entertained idea).
Hume considers a number of revealing examples: We often have no determinate idea of large numbers and merely use the impression of the name or word and attach to it our general habit and its feeling of understanding. In the same way we have no clear image for our complex ideas, such as church, negotiation, or conquest. Yet we feel we understand them in virtue of the felt impetus associated with the custom or habit connected with the impression of the word. An entire verse of poetry, though we can't recall it at the moment, can be brought back to us in a moment by one word (and we can feel the fullness, as it were, of the impetus attached to this word in our feeling of having the poem at the tip of our tongue.) Hume marvels at our ability to bring up relevant ideas at appropriate times without having a clear idea how we do so, calling this genius a magical faculty of the soul. In each of these cases the cognitive work is not done by the representative content of the idea (in many cases there is none besides the impression of the word), but by the mysterious causal force behind our idea and felt only as its impetus, or force and vivacity. When one considers just how much of our cognition involves these types of general ideas, it seems more appropriate to say that it is the unknown sources of the connections of our ideas that correspond to the felt force or impetus that do all the heavy lifting in cognition, not the ideas. The representational content in Hume’s Representational Theory of Mind is almost irrelevant.
To summarize, then: 1. Apart from the representational content of an idea there is another component: its force and vivacity, its impetus. 2. The impetus of ideas is felt, part of the phenomenology of the idea, though it is distinct from the content of the idea and is not itself another idea. 3. The impetus of ideas, as the name suggests, is active, is connected with habit or custom, and directs the production and flow of ideas. 4. Since the meaning of a general idea is a custom, the un-represented meaning of an idea is its impetus, which is distinct from its definition, or list of instances, or explicit rules for producing these instances. One cannot, it seems, look to Hume, for an example of the power of a no frills Representational Theory of Mind, as Fodor suggests. Once one sees the way ideas actually do their work for Hume, it becomes clear that they are merely tokens, along for the ride in the real business of the transmission of the force or impetus that thought involves. A consideration of Hume’s view of abstract ideas suggests that the real content of thought lies within the impetus, not the idea. Indeed if one takes seriously Hume’s suggestion that the source of the force and vivacity associated with an impression is the causal interaction with the world, then the everlasting universe of things is indeed flowing through us with the transmission and channeling of an impetus. And if an impetus originates in an impression and transmits the form of the object, the secret strength of things, which governs thought, and to the infinite dome of heaven is as a law, does indeed inhabit thee.
 Jerry Fodor, Hume Variations, Oxford University Press, 2003.
 “ALL the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and IDEAS. The difference betwixt these consists in the degrees of force and liveliness, with which they strike upon the mind, and make their way into our thought or consciousness. Those perceptions, which enter with most force and violence, we may name impressions: and under this name I comprehend all our sensations, passions and emotions, as they make their first appearance in the soul.” Book I, Part I, Section 1: A Treatise of Human Nature. (All quotes are from the Selby-Bigge edition.)
 “The first circumstance, that strikes my eye, is the great resemblance betwixt our impressions and ideas in every other particular, except their degree of force and vivacity.” Treatise. I, I, 1.
 “I would willingly establish it as a general maxim in the science of human nature, that when any impression becomes present to us, it not only transports the mind to such ideas as are related to it, but likewise communicates to them a share of its force and vivacity. Now 'tis evident the continuance of the disposition depends entirely on the objects, about which the mind is employ'd; and that any new object naturally gives a new direction to the spirits, and changes the disposition; as on the contrary, when the mind fixes constantly on the same object, or passes easily and insensibly along related objects, the disposition has a much longer duration.” Treatise. I, I, 8.
 “All the perceptions of the mind are of two kinds, viz. impressions and ideas, which differ from each other only in their different degrees of force and vivacity.' Our ideas are copy'd from our impressions, and represent them in all their parts. When you would any way vary the idea of a particular object, you can only increase or diminish its force and vivacity. If you make any other change on it, it represents a different object or impression. The case is the same as in colours. A particular shade of any colour may acquire a new degree of liveliness or brightness without any other variation. But when you produce any other variation, 'tis no longer the same shade or colour. So that as belief does nothing but vary the manner, in which we conceive any object, it can only bestow on our ideas an additional force and vivacity. An opinion, therefore, or belief may be most, accurately defined, A LIVELY IDEA RELATED TO OR ASSOCIATED WITH A PRESENT IMPRESSION.” Treatise. I, I, 7.
 The full quote reads: “An idea assented to feels different from a fictitious idea, that the fancy alone presents to us: And this different feeling I endeavour to explain by calling it a superior force, or vivacity, or solidity, or firmness, or steadiness. This variety of terms, which may seem so unphilosophical, is intended only to express that act of the mind, which renders realities more present to us than fictions, causes them to weigh more in the thought, and gives them a superior influence on the passions and imagination. I confess, that 'tis impossible to explain perfectly this feeling or manner of conception. We may make use of words, that express something near it. But its true and proper name is belief, which is a term that every one sufficiently understands in common life.” Treatise I, I, 7.
 This, doubtless, is what makes it attractive to Fodor.
 “A particular idea becomes general by being annex'd to a general term; that is, to a term, which from a customary conjunction has a relation to many other particular ideas, and readily recalls them in the imagination. The only difficulty, that can remain on this subject, must be with regard to that custom, which so readily recalls every particular idea, for which we may have occasion, and is excited by any word or sound, to which we commonly annex it. The most proper method, in my opinion, of giving a satisfactory explication of this act of the mind, is by producing other instances, which are analogous to it, and other principles, which facilitate its operation. To explain the ultimate causes of our mental actions is impossible. 'Tis sufficient, if we can give any satisfactory account of them from experience and analogy.” Treatise I, I, 7.
 “The fancy runs from one end of the universe to the other in collecting those ideas, which belong to any subject. One would think the whole intellectual world of ideas was at once subjected to our view, and that we did nothing but pick out such as were most proper for our purpose. There may not, however, be any present, beside those very ideas, that are thus collected by a kind of magical faculty in the soul, which, tho' it be always most perfect in the greatest geniuses, and is properly what we call a genius, is however inexplicable by the utmost efforts of human understanding.” Treatise I, I, 7.
 Indeed, we almost never employ particular impressions or ideas, merely as particular. Even when we look at a particular object at an instant, the impression at that moment, from that point of view, functions in cognition only through calling up other ideas of the object from other points of view and other sensory modalities.