by presupposing that the earth revolves around the sun, despite the intuitive evidence to the contrary. Kant sees himself as enabling progress in metaphysics. By metaphysics Kant means an account of non-empirical truths, that is, propositions that are necessarily true and whose truth we can establish without recourse to particular experiences of the world. He claims that we can only give such an account by presupposing, counter-intuitively, that objects conform to our a priori cognition of them, rather than the other way around. This claim is more readily expressed by saying that the structures of cognition constitute general features of objects or, as Kant himself puts it, that “we can cognize of things a priori only what we ourselves have put into them” (Bxviii). Besides constituting the objects of our experience, the same basic structures also constitute ourselves. So, although he is not consistently clear about this, Kant ends up with the view that subject and object are two interdependent poles in a single structure that constitutes the origin of meaningful experience. The task of philosophy is to analyze and spell out this underlying origin.
1.2 Intuitions and concepts
Cognition, says Kant, has two stems. On the one hand, we are receptive to sense data. Objects affect our sensory surfaces and give rise to a mostly unstructured “manifold” of sense impressions that means nothing by itself, but is a necessary element of any experience of an object. Kant calls our capacity to be affected by objects our “sensibility,” and he calls this mental content “intuitions.” A book on the desk or a familiar face, for example, affect our sense surfaces and give us a manifold of visual or tactile sense data including colors, lines, lighting, smoothness, and so on. This manifold resembles what William James called a “blooming, buzzing confusion” in his Principles of Psychology. It is mostly unstructured, but not entirely, for the sense data present themselves in a temporal sequence and in a spatial arrangement. The spatial and temporal order may be vague at first, but at least we have a sense that the orange patch is distinct from the brown patch and both are distinct from myself, because we intuit them at different moments and as located in different places. Kant argues that all intuitions must come in some temporal sequence, and all intuitions of objects distinct from us – that is, objects in the world, as opposed to our own thoughts – must present themselves in some spatial arrangement. A rough intuition of space and time, then, underlies all our sense data.
On the other hand our mind also actively structures experiences. We do this by organizing mental content according to concepts. A concept is a rule for recognizing a given intuition or a set of already cognized objects as an instance of a general type. Kant calls our capacity for spontaneously ordering a manifold and recognizing it under a general type the “understanding.” The understanding organizes the orange and brown patches and lines given in intuition as edges of a compact, colored object on a smooth surface, and recognizes it as a book on a desk. Just as space and time underlie all intuition, the understanding has some basic concepts that are required for all active structuring of mental content. Kant thinks, for example, that without basic notions of quantity (such as “one” or “many”), of negation, existence, or substance (a thing can persist as the same while some of its properties change), our understanding could never get off the ground. Kant produces a table of such basic concepts, which he calls the “categories.” The details of this table and Kant’s method for producing it may be challenged. But the overall point is well taken. A cognizer can certainly have experiences without some of our concepts. We can imagine a cognizer who lacks the concept of a book, of food, money, or whatever. But the categories are so fundamental to our cognition that without them no object-recognition, cognition, or experience is possible.
Obviously an unstructured manifold of intuitions is not yet an experience of anything. Less obviously, using concepts by themselves without applying them to intuitions also does not amount to an experience of anything. Cognition requires both stems. Kant puts this eloquently in a famous passage:
Thoughts without content are empty, intuitions without concepts are blind. . . . The understanding is not capable of intuiting anything, and the senses are not capable of thinking anything. Only from their unification can cognition arise. (A51)
This two-stem feature of Kant’s theory of cognition is fairly radical. Most philosophers prior to Kant think that a sense impression of a book and the concept of a book are the same kind of mental content. Hume, for example, thinks that they differ only insofar as the sense impression is more vivid than the concept, which is an attenuated and modified copy of the original impression. Leibniz, by contrast, thinks that the concept is clearer and more distinct, while sense impressions are vague and imprecise instances of conceptually determined experience. Kant’s reasons for claiming that intuitions and concepts cannot be reduced to one another derive mostly from his older argument about “incongruent counterparts,” which are pairs of objects that are conceptually equivalent, but differ perceptually. Regardless, in his Critique he focuses much of his analysis on explaining how intuitions and concepts are brought together in consciousness to produce objective experience. And that is where his view becomes truly groundbreaking.
We saw above that metaphysics, for Kant, consists of non-empirical knowledge, which he also calls a priori knowledge, and that Kant is trying to explain how and to what extent such knowledge is possible. According to the two-stem view of cognition, Kant is committed to saying that metaphysical knowledge must consist of a priori intuitions and a priori concepts. Kant believes that there are such intuitions and concepts. In fact, they are precisely the important basic structures we just outlined – space and time for intuitions and the categories for concepts. This a priori mental content ultimately grounds all possible metaphysical knowledge.
Kant’s argument that space, time, and the categories are a priori is fairly straightforward. All sense data are given as spatial and temporal (except for sense data that the mind gives to itself, which are only temporal). Since we need to have a representation of space and time in order to be given any sense data at all, we cannot derive our representation of space and time from what is given to us. Space and time are thus a priori intuitions. They cannot be concepts, because, as quoted above, Kant states explicitly that the understanding, that is, the faculty of concepts, “is not capable of intuiting anything.”
Moreover, space and time display some crucial hallmarks of intuitions. For example, unlike concepts they are not general terms that have a lot of instances falling under them. Different spaces or times are all parts of the same single space and time, not exemplars or instances of it. Kant’s argument that the categories are a priori is similar. Since they are necessary preconditions for having any experience at all, we cannot derive them from experience. I cannot get my concepts of existence or unity from my visual and tactile experience of a book, because I must be able to conceive of single, existing things in order to have an experience of the book in the first place. But if the categories cannot be derived from experience, they are not empirical (“empirical” just means “derived from experience”) and must be a priori.
If space, time, and the categories are not derived from experience, they must come from somewhere else. Kant thinks that they are innate in the human cognitive apparatus (and non-humans as well, if any of them are cognizers like us). Kant recognizes clearly that as subjective structures space, time, and the categories are specific to our experience. Space, he writes:
is nothing other than the subjective condition of sensibility, under which alone outer intuition is possible for us. . . . We can accordingly speak of space, extended beings, and so on, only from the human standpoint. If we depart from the subjective condition under which alone we can acquire outer intuition . . . then the representation of space signifies nothing at all. (A26, emphasis added)
He makes analogous claims about time (A34–5). And Kant also acknowledges that there could be an altogether different kind of cognizer, whose mind would not require concepts at all. He envisions, for example, a “divine understanding, through whose representation the objects would themselves at the same time be given, or produced” and notes that for such
