Back from the Coast and ready to tackle the most difficult part of the Critique. [One of the perennial problems in teaching the Critique is that the hardest part comes so near the beginning of the book. A week on the Prefaces and Introduction, a second week on the Transcendental Aesthetic, and whammo, you are right there in the Deduction. It never seems fair.]
Recall that Kant introduces the Categories in an effort to counter Hume's devastating critique of causal inference. Newton's Laws and all the rest of natural science are intended to apply to objects in space. Even after Kant has given up for all time any claims on knowledge of things as they are in themselves, he is still left with the problem of showing that we can have causal knowledge of things as they appear to us in space [and time.] In a very deep and very difficult move, Kant tackles this challenge by going all the way back to Descartes' foundational premise, Cogito, "I think," and extracting from it what he thinks is the correct conclusion, not the mistaken inferences that Descartes draws in the remainder of the Meditations.
The key is an idea that Kant calls "the unity of consciousness." The mind is affected by things in themselves, and produces a diversity or manyness or manifold of sense contents on which it imposes the form of space. But in some way that requires explanation, it is able to hold this diversity of sense contents together in a single thought. I think these many contents as my thoughts. As Kant says, significantly revising Descartes' cogito, "The 'I think" attaches to all my representations." This is the radically subjectivist implication of the Epistemological Turn. It is always I who think the thoughts in my mind. But this is puzzling. How can a multiplicity of representations be thought as a unity? Kant's answer is that the mind runs through them and holds them together. It synthesizes them, by bringing them under the Categories lying ready in the mind. The Categories are thus, Kant says, conditions of the possibility of experience in general, where the term "experience" [erfahrung] means "empirical knowledge."
But "running through a manifold and holding it together in one concept" is not exactly transparently clear as an explanation for the unity of consciousness. Kant thought of this as his signal contribution to the Theory of Knowledge, and so it was, suitably fleshed out and explicated, but unbeknownst to Kant, Hume had already identified the problem [this is in a part of the Treatise that Kant never read, and that Beattie did not quote], and as usual, Hume's account of the problem is much simpler and clearer than Kant's. Here is what Hume says, in Book One, Part iv, Section ii of the Treatise, with the intriguing title, "Of Scepticism With Regard to the Senses." He is trying to explain how we form the idea of the continuing identity of an object through time:
"First, As to the principle of individuation; we 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 existences are entirely distinct and independent.
Since then both number and unity are incompatible with the relation of identity, it must lie in something that is neither of them. But to tell the truth, at first sight this seems utterly impossible. Betwixt unity and number there can be no medium; no more than betwixt existence and non-existence. After one object is suppos’d to exist, we must either suppose another also to exist; in which case we have the idea of number: Or we must suppose it not to exist; in which case the first object remains at unity."
Further to clarify this central notion of the unity of consciousness, let me repeat here a little thought experiment that I used in Kant's Theory of Mental Activity, borrowing it from Norman Kemp Smith, who got it from William James, who got it from Franz Brentano. Here is how James puts it: "Take a sentence of a dozen words, and take twelve men, and tell to each one word. Then stand the men in a row or jam them in a bunch, and let each think of his word as intently as he will; nowhere will there be a consciousness of the whole sentence." [Principles of Psychology, Volume I, p. 160.]
Hokey though it may sound in the telling, I would actually write a sentence on two pieces of paper, tear one of them into one-word segments, hand the first piece to one person and each of the torn segments to separate people, and have them stand in front of the room and read what was on their piece of paper. I used James Thurber's great six-word sentence, "The unicorn is a mythical beast." The point is that although "The," "unicorn," "is," "a," "mythical" and "beast" were each in someone's consciousness, they were united only in the consciousness of the person who held the first piece of paper. I know, I know, Show and Tell is not exactly appropriate for a graduate course at Harvard University, but it worked very nicely when I first tried it in the Spring of 1960.
Because this is important, and not at all easy to grasp, I am going to go over it carefully several times, at the risk of seeming tedious. Kant starts, in the Inaugural Dissertation, with the traditional view that it is possible, by the use of the power of Reason, to gain knowledge a priori of independent reality ["of things as they are in themselves," or, in the famous phrase, dinge an sich. Jokey side note: in 1954, when I bought a tiny motorcycle in Oxford for my trip to the Continent, I called it the "ding nicht an sich" because it was a phenomenal motorcycle. I was twenty at the time.]
In a major concession to the subjectivist, or sceptical, or empiricist side of the great debate, Kant gives up the belief that we can have knowledge a priori of independent reality, and introduces the notion of mind-dependent forms of intuition to explain how the mind can have genuine knowledge of things as they appear to us in space and time ["phenomena," as he calls them.] Almost immediately, he encounters Hume's criticisms of causal inference, which, because they apply to things in space and time, call into question even the limited claims Kant has made for Newtonian science. In response, he makes two moves: First, he introduces the notion of Pure Concepts of Understanding, or Categories, which, like Space and Time, are mind-dependent forms -- this time of conception, not of intuition -- that the mind imposes on the spatio-temporal manifold of sense contents. And Second, he argues that we can know, a priori, that the Categories find application in experience [the so-called Deduction of the Categories] because only by applying them to the manifold of sense-contents in the process of synthesis can we bring them to consciousness at all.
Let us be clear about the dialectical situation we now confront, if I may use that term. Imagine Kant engaging Hume in a public debate, before an audience of philosophers. Kant begins by claiming [The Inaugural Dissertation] that the Wolffian version of Leibnizean metaphysics is true. [That is Christian Wolff, of course.] Hume replies with sceptical doubts about the possibility of having genuine knowledge [universal and necessary] of independent reality. Kant concedes the point, but introduces the notion that space and time are mind-dependent forms of sensuous intuition, and hence ground the knowledge claims of Geometry. Hume returns to the attack, arguing that Newton's physical theories [and all other physical theories, of course] apply to objects in space and time, and since representations in space and time that are distinguishable from one another are separable at least in the imagination, so that we can at least imagine that one should occur without the other, there are no necessary connexions between objects in space and time Hence, the knowledge claims of Newtonian physics must be given up. Now Kant comes back at Hume with his boffo rejoinder.
"You deny that we can have knowledge of independent reality, and you deny that we can have knowledge of causal connections between objects even if we recognize that those objects are merely appearances in space and time. But surely you must grant that you are conscious, and that your conscious has a unity. You cannot coherently deny that minimal claim, because you implicitly acknowledge it in the very act of asserting the proposition. But I can show you that the condition of the possibility of that unity of consciousness is the applicability of the categories to the manifold of perceptions, and the categories whose applicability is a condition of consciousness itself include none other than the category of cause and effect.
Whereupon Hume retires from the field of elenchic battle and Kant claims the day.
That is what is really going on in the dense pages of the Transcendental Analytic. But several crucial questions remain as yet unanswered. And we may be sure that Hume would ask them, once he had recovered his customary aplomb.
First, if our knowledge claims are true only under the condition that they are restricted to the realm of mere appearances, in what recognizable sense can they be described as objective, universal, and necessary? Second, where did you get these so-called Categories from? The derivation from the Table of Functions of Unity in Judgment is totally ad hoc and unsatisfactory in several different ways. Third, and most fundamental, you describe the Categories as forms of synthesis, and you describe synthesis as the act of running through and holding together" a manifold of sensuous intuition. But "running through and holding together" is a metaphor, at best. What is this activity called "synthesis," and how does it impart unity to a multiplicity? Until you can answer these questions, you have proven nothing.