at all” (B145).
Kant’s view that objects are intelligible to us only from the human standpoint is called his “transcendental idealism.” Idealism is the view that objects depend on our minds. Kant’s idealism is “transcendental,” because on his view this dependence only shows itself insofar as we consider the basic constitutive structures of the human standpoint. This is a specific, limited philosophical perspective that we only adopt when we are doing transcendental philosophy, that is, a philosophical analysis of the constitution of experience. For all ordinary and scientific purposes, we necessarily remain within the human standpoint, and must therefore be realists about objects. In fact the main lesson from transcendental philosophy is that the objects of experience have a lot of universal and necessary features that we can know about. Kant therefore pairs his transcendental idealism with a clear commitment to empirical realism. For example, he insists on:
the reality (i.e., objective validity) of space in regard to everything that can come before us externally as an object, but at the same time the ideality of space in regard to things when they are considered in themselves through reason. (A28)
In this respect, Kant’s transcendental idealism differs from the views of Descartes or Berkeley, who doubted the reality of external objects from within the human standpoint.
1.3 The transcendental deduction
With this we have come to the crux of Kant’s critical philosophy, and also to the beginning of his most important and most difficult argument, the “transcendental deduction.” Obviously space, time, and the categories constitute experience for us. We cannot have experience any other way. But why should we think that the forms of our sensibility and the basic concepts that we must rely on as a matter of the finitude of our minds are an appropriate fit for the way things really are? What rational justification can we have for taking our so-constituted experience to be objective? The purpose of Kant’s transcendental deduction is to explain how these subjective conditions can constitute objective experience. Kant actually produces two versions of this long and complicated argument, one for each edition of the Critique of Pure Reason, accordingly known as the A-deduction and the B-deduction. They differ in the details, but have the same overall strategy. Like the table of categories (and much else in the Critique), some of the detailed claims Kant makes in the deduction can be challenged, and it is debatable whether the overall argument works. Scholars even debate what the structure of the deduction is, whether it consists of a single argument or two independent ones, what the premises are, and so on. However, all agree that the basic insights that drive the deduction are groundbreaking and define the development of post-Kantian philosophy for well over a century.
The deduction hinges on the notion of the unity of our consciousness. As is clear from the two-stem view, any cognition or experience of objects requires that the mind bring together given intuitions and concepts. This can only occur if the mind itself is unified. If one person has the sense data of the orange patch in her mind, while another has the concept of a book in hers, neither of the two perceives the book. All relevant mental content must be held and processed in the same, single mind. This much is obvious. Kant’s genius lies in his realization that the requisite unity of consciousness is more complex and structured than others had realized, and he gives a stunningly subtle analysis of this complexity.
The unity of consciousness is not a passive state. The mind is not a receptacle, like a bowl in which we mix several ingredients to make a cake. Rather, Kant argues that the unity of consciousness is an active mental process of unifying. He calls this process synthesis, and he claims that the unity of consciousness that makes cognition possible is a synthetic unity. Synthesis is required even for the elementary task of intuiting a manifold of sense data. To have such a manifold, the mind needs to be aware of each element of the manifold, and it needs to be aware of them (or at least to represent them at a pre-cognitive level) as distinct from one another. To intuit an orange patch and a brown patch requires that I can represent the orange and the brown as well as their difference. The representation of their difference, and hence the representation of their “two-ness,” or manifold, is more than the sum of two distinct representations. It requires the mind to apprehend one datum and keep it present, or reproduce it, while it apprehends the second. To further recognize these sense data as a book on a desk, or even just as a manifold of colored objects, the mind runs through this manifold of intuitions and organizes them according to a rule that constitutes the concept of that object. Kant therefore speaks of a “threefold synthesis” of apprehension, reproduction, and recognition. The A-deduction goes through this threefold synthesis in great detail. The B-deduction goes through it quickly and focuses on trying to spell out the important philosophical claims that follow from the recognition of this fundamental synthetic process.
The first consequence relates to the object side of the synthesis, and addresses the central question of the deduction. Since consciousness of anything is an achievement of synthesis, the structures implicit in this synthesis must be basic determinations of anything that we could encounter as an object of consciousness. That is to say that the very notion of “object” has the structures that accrue to it in synthesis. Kant goes on to claim, more or less plausibly, that these structures are precisely the categories whose objectivity is in question, and that therefore the categories are objectively valid. Note that this conclusion is substantially stronger than the previously established claim that the categories are a priori concepts. One could think that concepts are a priori, that is, not derived from experience, and that we cannot help but use them in experiencing, but still doubt that they characterize intrinsic features of objects. This is what Hume thought about causation, for example. According to Hume, we cannot get the concept of causation from experience, so it is non-empirical, and we use it all the time in judging matters of fact. But we cannot rationally justify this use. One of Kant’s big goals in the Critique is to find a solution to Hume’s skepticism about the rational legitimacy of our a priori concepts. Kant therefore mentions Hume as one of his targets as he introduces the deduction (B128). In the deduction Kant concludes from the necessity of synthesis that we cannot even make sense of the notion of an object aside from categorical determinations. We cannot rationally entertain Hume’s skeptical worry about objects, because the fully understood notion of an object already answers the skepticism.
A second consequence has to do with self-consciousness. Just as a synthesis is required to represent a manifold, a synthesis is also a necessary condition for self-consciousness. In particular, Kant focuses on a kind of self-consciousness that he calls “apperception.” Apperception is my consciousness that a particular thought or cognition is mine. When I see the book on the table, I am conscious that I am perceiving it, or at least I can become conscious of this. In general, I can apperceive any act of my consciousness, for otherwise it would not count as an act of my consciousness. To apperceive distinct conscious acts is to be conscious of myself as the same consciousness in each apperception, and this apperception of the sameness of the self across different self-conscious acts is a further apperceptive act beyond the particular ones. Kant calls this background awareness of the unity of consciousness the “synthetic unity of apperception,” because it presupposes a synthesis. In fact Kant thinks that this is the same synthesis that makes consciousness of objects possible. The same necessary conditions apply to object-consciousness and self-consciousness. Kant’s argument in the transcendental deduction, therefore, establishes an essential connection between objective features of the world and self-consciousness.
In more general terms, Kant’s deduction undermines the starting point of both Humean empiricism and Cartesian skepticism. On the empiricist side, Hume thought that all mental content derives from original impressions. Kant shows that we cannot presume that the mind is simply given discrete impressions of the world. By the time the mind can represent even simple impressions from given objects, it has already engaged in extensive conceptual processes that introduce a robust a priori structure into the world of possible experience. Descartes thought that his famous cogito, that is, subjective self-consciousness, is intelligible in complete isolation from any possible
