Hume has an elaborate account of the several innate mental propensities that, working together, lead us to believe in the continued and independent existence of objects, but over and above this story, he realizes that he must make some sense out of the very notion of the continued and independent existence of a single thing that is nonetheless spread out over space and time. What can it possibly mean to say that the face the baby sees the second time is the identically same face as the face the baby sees the first time? What, in general, does it mean to say that something that exists over time, and what is more changes as time passes, is nevertheless the same thing? Hume is quite correct in supposing that this is the most fundamental notion we employ in making sense of our experience, and he is also quite correct in supposing that it is a puzzle.
Aristotle, you will recall, dealt with this problem by distinguishing between the essence of a thing and its accidents. The girl and the woman she becomes are one substance, having a single essence, even though the accidental properties – height, weight, distribution of hair, and the rest – change as time passes. Without some notion of “the same thing,” we cannot even coherently say that “my friend returned the book she borrowed from me,” or “I am not so hot as I was yesterday, when the temperature reached 90 degrees,” or “Is this a dagger I see before me?”
Those of you who followed my tutorial on Kant’s First Critique will recall the passage I quoted from Hume, a passage from this very section we are now considering. Here it is again:
“[W]e may observe, that the view of any one object is not sufficient to convey the idea of identity. For in that proposition, an object is the same with itself, if the idea express’d by the word, object, were no ways distinguish’d from that meant by itself; we really shou’d mean nothing, nor wou’d the proposition contain a predicate and a subject, which however are imply’d in this affirmation. One single object conveys the idea of unity, not that of identity.
“On the other hand, a multiplicity of objects can never convey this idea, however resembling they may be suppos’d. The mind always pronounces the one not to be the other, and considers them as forming two, three, or any determinate number of objects, whose existence are entirely distinct and independent.”
Hume’s offers a complex and fascinating explanation for our notion of identity. [Note that he must explain the idea itself as well as our tendency to believe that we are presented with objects that are identical over time and despite changes in their properties.] It involves appeal to what he calls a “distinction of reason” and also to a new mental tendency he discovers, the tendency we have to associate together, and confuse, different actions of the mind that resemble one another. It seems that we not only associate together pairs of impressions that resemble one another [this impression of fire and this impression of heat, that impression of fire and that impression of heat, a third impression of fire and a third impression of heat, and so forth]; we also associate together pairs of mental activities that resemble one another. Here is the story, simplified as much as I am able.
First of all, when the mind is presented with a single impression, it forms the idea of a unity. When the mind is presented with a succession of perfectly resembling impressions [as when I stare at a picture fixedly for a while], we are aware only of a single unchanging object of perception, and we are not aware of any passage of time, so the notion of unity is appropriately applied. But suppose that other objects in our field of vision are changing during this succession of perfectly resembling impressions. In that case, we become aware of the passage of time as a consequence of these changes in other parts of our visual or auditory field. Now, by a “distinction of reason,” we suppose the unchanging perception to participate in the temporal succession of the series of changing perceptions that occupy the same field of perception. And so we say of that unchanging perception that it is both a unity [because considered simply in itself, there is no multiplicity in it] and also a multiplicity, because considered in relation to the surrounding changing perceptions, it continues through time, and thus is really not a single perception but a succession of perfectly resembling perceptions. So it is that by conflating these two, we arrive at the [internally contradictory but nevertheless irresistible] notion of identity, which is the unity of a multiplicity. [You see the connection here with Kant’s notion of synthesis of the manifold -- this was the idea that occurred to me when I was writing my undergraduate General Examinations at Harvard in 1953, and which eventually became central to my book, Kant’s Theory of Mental Activity and my journal article “Hume’s Theory o Mental Activity.”]
The simple, universal notion of an object – which is to say, something that lasts through time, and yet is the same thing during that time, is, if Hume is right, at base a confusion, a contradiction, a logical absurdity. Hume knows that, and he means it. The mind is so constituted that it inevitably forms and employs this notion, and yet reason can give no coherent account of it.
But of course experience only rarely affords us the luxury of a sequence of perfectly resembling impressions distinguishable one from the other only because other parts of our perceptual field are changing during that sequence. In two ways, experience falls short of the ideal [which, let us always recall, gives rise to an idea of identity that is, strictly speaking, an absurdity]. Hume has a story to tell about each.
First of all, our experience even of perfectly resembling impressions is gappy, as I have already noted. I look this way and that in my study [this is Hume’s example], and I have a series of perceptions of the books on the shelves. The perceptions are perfectly resembling – nothing changes on the shelves in the few minutes that I am looking around – but the resembling perceptions are interrupted by the fact that I look now in one direction and now in another. But the action of the mind in running over an unbroken sequence resembles the action of the mind in running over a sequence in which there are gaps. Hence, by a natural propensity, the mind associates the two actions together, and confuses them, imputing to the gappy sequence the same [inherently confused] idea of identity that it had previously imputed to the unbroken sequence.
But to complete this association of, and consequent confusion of, two different actions of the mind I must now posit the continued existence of the object when I am not observing it, which, to be absolutely precise means assuming the existence of a series of intervening and unperceived perceptions that, were they perceived, would constitute an unbroken sequence of perfectly resembling impressions. In short, the baby supposes that the father’s face goes on existing when it is out of sight, looking, when it is not seen, exactly as it would look if it were seen.
This is an extraordinarily complicated mental process, all in the service of producing the simplest imaginable idea, that of an object.
But that is the very least of what is actually involved, for just as we are rarely in possession of an unbroken sequence of perfectly resembling impressions, so we are also quite often not in possession even of a broken sequence of perfectly resembling impressions. And yet we impute identity even in cases in which the impressions undergo changes. Tomorrow we shall see what Hume has to say about that.
Incidentally, this is as good a time as any to recommend an old book on this subject from which I learned a good deal. It is by the English philosopher H.H. Price and it is called Hume’s Theory of the External World.