In the transcendental aesthetic of the Critique of Pure Reason, Kant claims that space and time are neither things in themselves nor properties of things in themselves but mere subjective forms of our sensible experience. Call this the Subjectivity Thesis.1 The striking conclusion follows an analysis of the representations of space and time. Kant argues that the two representations function as a priori conditions of experience, and are singular "intuitions" rather than general concepts. He also contends that the representations underwrite some non-trivial a priori cognition of the objects of sensible experience. The Subjectivity Thesis is then presented as an immediate consequence:
Space represents no property at all of any things in themselves nor any relation of them to each other. . . . For neither absolute nor relative determinations can be intuited prior to the existence of the things to which they pertain, thus [neither] can be intuited a priori.(A26/B42)
Time is not something which exists of itself, or which inheres in things as an objective determination. . . . [Were it] a determination or order inhering in things themselves, it could not precede the objects as their condition, and be known and intuited a priori by means of synthetic propositions.(A32–33/B49)
Kant's argument for the Subjectivity Thesis seems to have the following structure: [End Page 355]
(ST1) No absolute or relational features of things in themselves can be cognized a priori.
(ST2) Some spatial and temporal features of objects of experience can be cognized a priori.
(ST) Spatial and temporal features are not features of things in themselves.
The major premise (ST1) denies the possibility of non-trivial a priori knowledge of things in themselves.2 Kant explains that no features of things in themselves could be cognized "prior to the existence of the things to which they pertain" (A26/B42). Though this construction is somewhat unusual, the idea appears simple enough. The claim seems to be that we cannot have knowledge of the constitution of a mind-independent entity before we know that it exists.3 On the assumption that such existence could not be established a priori (that is, "absolutely independently of all experience"4), it follows that we cannot have substantive a priori knowledge of the constitution of mind-independent entities. Such reasoning will be accepted by many philosophers who hold that knowledge cannot be grounded in an accidental correspondence between thought and mind-independent reality, but rather requires a suitable causal connection to what is known.
Note that Kant's exclusion of a priori knowledge is intended to apply only to things in themselves. He claims by contrast that some spatiotemporal features of objects of experience are knowable a priori. It is a central claim of the Transcendental Aesthetic and the KrV as a whole that some non-trivial knowledge of all objects of experience does precede knowledge of whether or not particular empirical objects exist.
At first blush, Kant's argument for the Subjectivity Thesis seems quite hopeless. One of its premises excludes a priori knowledge of things as they are in themselves. The other premise lays claim to a priori knowledge of objects of experience. Suppose then, for the sake of argument, that we cannot have either empirical or a priori knowledge of things in themselves, and so (ST1) is true. Suppose also that (ST2) is true, and we really can have a priori cognition of whatever precise spatial and temporal features of objects of experience Kant has in mind in the Transcendental Aesthetic. And suppose, finally, that things in themselves just happen to have spatial and temporal features too, so (ST) is false. This simple model exhibits the invalidity of the argument as reconstructed above.
Kant's contemporary H. A. Pistorius seems to have been to first to argue that the KrV's central argument for transcendental idealism does not succeed in ruling out the "intelligible and thinkable" thesis that the spatiotemporal form of appearances agrees with the order of things as they really are. In the nineteenth century, Trendelenburg insisted in the same spirit on a crucial gap in the argument. Kant "hardly considered...