Hume is about to introduce the second component of his theory -- the component in a sense intended to echo Newton -- but first he pauses to distinguish between two faculties of the mind, Memory and Imagination, and in doing so, he reveals some serious, if not fatal, limitations to the simple Copy Theory of Ideas that he has laid out in the opening pages of Book I, Part I. Both Memory and Imagination enable us to call to mind previous perceptions -- in the case of Memory, accurately and in conformity with the sequence and arrangement in which they first presented themselves to us, in the case of Imagination without such limitations, although of course it is still the case that Imagination can call to mind idea-copies only of those simple impressions we have previously experienced. How can we distinguish between things we remember and things we imagine? By the degree of their force and vivacity, Hume replies. The ideas called up by Memory are more lively and strong than those called up by Imagination.
This is pretty clearly the only answer available to Hume, given the elements he has thus far laid down. All impressions and ideas are contents of consciousness, and the only distinctions he can at this point draw with regard to them are whether they are simple or complex and whether they are forceful and vivacious or not. But this answer is manifestly wrong. Everyone has had experience with faded and listless memories, if I may speak in that way, and with lively, forceful, vivid daydreams that are nevertheless plainly the product of Imagination. Hume himself knows this quite well, and elsewhere remarks on the contrast between a dry, lifeless historical account of an event and a vivid, lively fictional account found in a novel. In a bit, I shall indicate how Hume very considerably complicates his story in order to deal with this and many other problems. To anticipate a bit, he is going to have to draw a distinction between the way the mind treats or thinks about or considers ideas that are called up by Memory as contrasted with those called up by Imagination, and this is going to involve talking about activities of the mind, not simply contents of the mind. But first, let us see how Hume introduces the other central element of his theory.
Hume pretty clearly understood Newton to be offering us a theory about a world of material objects, of bodies, connected with one another and interacting with one another by means of a single force, gravity. His big idea, if I may put it that way, was to conceptualize Moral Philosophy as having the same structure. Perceptions -- impressions and ideas -- are the elements, corresponding to Newton's bodies in motion. And as the force connecting them, Hume proposes association. I am going to quote at length the first paragraph of the section entitled "Of the connexion or association of ideas," since Hume says it more clearly and concisely than I can.
"As all simple ideas may be separated by the imagination, and may be united again in what form it pleases, nothing wou'd be more unaccountable than the operations of that faculty, were it not guided by some universal principles, which render it, in some measure, uniform with itself in all times and places. Were ideas entirely loose and unconnected, chance alone wou'd join them; and 'tis impossible the same simple ideas should fall regularly into complex ones (as they commonly do) without some bond of union among them, some associating quality, by which one idea naturally introduces another. This uniting principle among ideas is not to be consider'd as an inseparable connexion; for that has been already excluded from the imagination: nor yet are we to conclude, that without it the mind cannot join two ideas; for nothing is more free than that faculty: but we are only to regard it as a gentle force, which commonly prevails, and is the cause why, among other things, languages so nearly correspond to each other; nature in a manner pointing out to every one those simple ideas, which are most proper to be united into a complex one. The qualities, from which this association arises, and by which the mind is after this manner convey'd from one idea to another. are three, viz. RESEMBLANCE, CONTIGUITY in time or place, and CAUSE and EFFECT."
Let us be clear what Hume is saying. Perceptions flood into our consciousness, of all degrees of vivacity and in endless variety. The mind has the ability to recall them in weakened form as ideas, and to combine them in any way it chooses, either mimicking the order and arrangement with which they entered our minds or arranging them in new and original orders. Nothing compels the mind to recall them in one way rather than another, to be sure. If that were so, then Logic would enable us to deduce a priori which ideas must go with which. But we observe that in fact the recall of impressions or previously apprehend ideas is not entirely random. Rather, certain ideas tend to be recalled together, and upon examination, we find that those ideas are recalled together that resemble one another, or are near to one another in time or place in our original experience of them, or else are connected as cause is to effect. This is not a deduction on Hume's part; it is an observation.
As he is drawing the Section to a close, Hume explicitly draws the analogy to between the "gentle force of association" and the gravitational attraction on which Newtonian physics depends. "Here is a kind of ATTRACTION, which in the mental world will be found to have as extraordinary effects as in the natural, and to shew itself in as many and varied forms." Newton had said, in the Principia, that he eschewed "hypotheses," by which he meant metaphysical speculations about the inner nature of things, contenting himself with exhibiting the laws governing the observable behavior of bodies. Hume echoes that forswearing of speculation, saying of association that "its effects are every where conspicuous; but as to its causes, they are mostly unknown, and must be resolv'd into original qualities of human nature, which I pretend not to explain."
But the parallel Hume seeks to draw between association and gravity is a false one, and a true understanding of what is going on in the Treatise requires that we be clear exactly in what ways the parallel breaks down. Gravity, whatever its sources or inner nature, really does move bodies closer to one another, in ways precisely predicted by Newton's Laws. But association, "gentle force" or no, does not move perceptions closer to one another. It affects the mind, not the perceptions. It is the mind that does the associating. To say that the mind tends to associate fire and heat together is to say that when I see fire, I form the idea of heat. The association is not in the ideas but in the mind.
Unlike Kant, who is quite comfortable talking about Faculties of the Mind -- Understanding, Reason, Imagination, Sensibility, and so forth -- Hume tries very hard not to use the language of Faculty Psychology, but he cannot avoid it, because the very foundation of his theory of knowledge and --as we shall see -- of morals requires him to spell out in considerable detail an account of what the mind does with its perceptual contents. In short, Hume is offering us a Theory of Mental Activity.
The Treatise is chock full of detailed accounts of all manner of mental functionings, but as we shall see, there are really only two big subjects that Hume wishes to tackle in Book I by means of his new theory of association. The first is Causation, and the second is Substance [or Material Objects.] In each case, Hume's discussion begins with a negative or sceptical argument, refuting the claims that philosophers have made about the subject under discussion. That is the part of Hume's text that is best known, although I think it is not the most interesting. After deploying his sceptical arguments, Hume starts again and asks, "Why do we, despite these negative arguments, come to believe that events and objects are causally connected, or to believe that there are independently existing, unified objects that persist through time?" He then gives an original and very imaginative psychological explanation for our beliefs, which, he insists, we are completely unable to give up despite the fact that they have no logical justification whatsoever.