With Kant's claim that the mind of the knower makes an active contribution to
experience of objects before us, we are in a better position to understand transcendental idealism. Kant's arguments are designed to show the limitations of our knowledge. The Rationalists believed that we could possess metaphysical knowledge about God, souls, substance, and so; they believed such knowledge was transcendentally real. Kant argues, however, that we cannot have knowledge of the realm beyond the empirical. That is, transcendental knowledge is ideal, not real, for minds like ours.
Kant identifies two a priori sources of these constraints. The mind has a receptive capacity, or the sensibility, and the mind possesses a conceptual capacity, or the understanding.
In the Transcendental Aesthetic section of the Critique, Kant argues that sensibility is the understanding's means of accessing objects. The reason synthetic a priori judgments are possible in geometry, Kant argues, is that space is an a priori form of sensibility. That is, we can know the claims of geometry with a priori certainty (which we do) only if experiencing objects in space is the necessary mode of our experience. Kant also argues that we cannot experience objects without being able to represent them spatially. It is impossible to grasp an object as an object unless we delineate the region of space it occupies. Without a spatial representation, our sensations are undifferentiated and we cannot ascribe properties to particular objects. Time, Kant argues, is also necessary as a form or condition of our intuitions of objects. The idea of time itself cannot be gathered from experience because succession and simultaneity of objects, the phenomena that would indicate the passage of time, would be impossible to represent if we did not already possess the capacity to represent objects in time.
Another way to understand Kant's point here is that it is impossible for us to have any experience of objects that are not in time and space. Furthermore, space and time themselves cannot be perceived directly, so they must be the form by which experience of objects is had. A consciousness that apprehends objects directly, as they are in themselves and not by means of space and time, is possible--God, Kant says, has a purely intuitive consciousness--but our apprehension of objects is always mediated by the conditions of sensibility. Any discursive or concept using consciousness (A 230/B 283) like ours must apprehend objects as occupying a region of space and persisting for some duration of time.
Subjecting sensations to the a priori conditions of space and time is not sufficient to make judging objects possible. Kant argues that the understanding must provide the concepts, which are rules for identifying what is common or universal in different representations.(A 106) He says, "without sensibility no object would be given to us; and without understanding no object would be thought. Thoughts without content are empty; intuitions without concepts are blind." (B 75) Locke's mistake was believing that our sensible apprehensions of objects are thinkable and reveal the properties of the objects themselves. In the Analytic of Concepts section of the Critique, Kant argues that in order to think about the input from sensibility, sensations must conform to the conceptual structure that the mind has available to it. By applying concepts, the understanding takes the particulars that are given in sensation and identifies what is common and general about them. A concept of "shelter" for instance, allows me to identify what is common in particular representations of a house, a tent, and a cave.
The empiricist might object at this point by insisting that such concepts do arise from experience, raising questions about Kant's claim that the mind brings an a priori conceptual structure to the world. Indeed, concepts like "shelter" do arise partly from experience. But Kant raises a more fundamental issue. An empirical derivation is not sufficient to explain all of our concepts. As we have seen, Hume argued, and Kant accepts, that we cannot empirically derive our concepts of causation, substance, self, identity, and so forth. What Hume had failed to see, Kant argues, is that even the possibility of making judgments about objects, to which Hume would assent, presupposes the possession of these fundamental concepts. Hume had argued for a sort of associationism to explain how we arrive at causal beliefs. My idea of a moving cue ball, becomes associated with my idea of the eight ball that is struck and falls into the pocket. Under the right circumstances, repeated impressions of the second following the first produces a belief in me that the first causes the second.
The problem that Kant points out is that a Humean association of ideas already presupposes that we can conceive of identical, persistent objects that have regular, predictable, causal behavior. And being able to conceive of objects in this rich sense presupposes that the mind makes several a priori contributions. I must be able to separate the objects from each other in my sensations, and from my sensations of myself. I must be able to attribute properties to the objects. I must be able to conceive of an external world with its own course of events that is separate from the stream of perceptions in my consciousness. These components of experience cannot be found in experience because they constitute it. The mind's a priori conceptual contribution to experience can be enumerated by a special set of concepts that make all other empirical concepts and judgments possible. These concepts cannot be experienced directly; they are only manifest as the form which particular judgments of objects take. Kant believes that formal logic has already revealed what the fundamental categories of thought are. The special set of concepts is Kant's Table of Categories, which are taken mostly from Aristotle with a few revisions:
Inherence and Subsistence
Causality and Dependence
While Kant does not give a formal derivation of it, he believes that this is the complete and necessary list of the a priori contributions that the understanding brings to its judgments of the world. Every judgment that the understanding can make must fall under the table of categories. And subsuming spatiotemporal sensations under the formal structure of the categories makes judgments, and ultimately knowledge, of empirical objects possible.
Since objects can only be experienced spatiotemporally, the only application of concepts that yields knowledge is to the empirical, spatiotemporal world. Beyond that realm, there can be no sensations of objects for the understanding to judge, rightly or wrongly. Since intuitions of the physical world are lacking when we speculate about what lies beyond, metaphysical knowledge, or knowledge of the world outside the physical, is impossible. Claiming to have knowledge from the application of concepts beyond the bounds of sensation results in the empty and illusory transcendent metaphysics of Rationalism that Kant reacts against.
It should be pointed out, however, that Kant is not endorsing an idealism about objects like Berkeley's. That is, Kant does not believe that material objects are unknowable or impossible. While Kant is a transcendental idealist--he believes the nature of objects as they are in themselves is unknowable to us--knowledge of appearances is nevertheless possible. As noted above, in The Refutation of Material Idealism, Kant argues that the ordinary self-consciousness that Berkeley and Descartes would grant implies "the existence of objects in space outside me." (B 275) Consciousness of myself would not be possible if I were not able to make determinant judgments about objects that exist outside of me and have states that are independent of the of my inner experience. Another way to put the point is to say that the fact that the mind of the knower makes the a priori contribution does not mean that space and time or the categories are mere figments of the imagination. Kant is an empirical realist about the world we experience; we can know objects as they appear to us. He gives a robust defense of science and the study of the natural world from his argument about the mind's role in making nature. All discursive, rational beings must conceive of the physical world as spatially and temporally unified, he argues. And the table of categories is derived from the most basic, universal forms of logical inference, Kant believes. Therefore, it must be shared by all rational beings. So those beings also share judgments of an intersubjective, unified, public realm of empirical objects. Hence, objective knowledge of the scientific or natural world is possible. Indeed, Kant believes that the examples of Newton and Galileo show it is actual. So Berkeley's claims that we do not know objects outside of us and that such knowledge is impossible are both mistaken.
In conjunction with his analysis of the possibility of knowing empirical objects, Kant gives an analysis of the knowing subject that has sometimes been called his transcendental psychology. Much of Kant's argument can be seen as subjective, not because of variations from mind to mind, but because the source of necessity and universality is in the mind of the knowing subject, not in objects themselves. Kant draws several conclusions about what is necessarily true of any consciousness that employs the faculties of sensibility and understanding to produce empirical judgments. As we have seen, a mind that employs concepts must have a receptive faculty that provides the content of judgments. Space and time are the necessary forms of apprehension for the receptive faculty. The mind that has experience must also have a faculty of combination or synthesis, the imagination for Kant, that apprehends the data of sense, reproduces it for the understanding, and recognizes their features according to the conceptual framework provided by the categories. The mind must also have a faculty of understanding that provides empirical concepts and the categories for judgment. The various faculties that make judgment possible must be unified into one mind. And it must be identical over time if it is going to apply its concepts to objects over time. Kant here addresses Hume's famous assertion that introspection reveals nothing more than a bundle of sensations that we group together and call the self. Judgments would not be possible, Kant maintains, if the mind that senses is not the same as the mind that possesses the forms of sensibility. And that mind must be the same as the mind that employs the table of categories, that contributes empirical concepts to judgment, and that synthesizes the whole into knowledge of a unified, empirical world. So the fact that we can empirically judge proves, contra Hume, that the mind cannot be a mere bundle of disparate introspected sensations. In his works on ethics Kant will also argue that this mind is the source of spontaneous, free, and moral action. Kant believes that all the threads of his transcendental philosophy come together in this "highest point" which he calls the transcendental unity of apperception.