This a super old essay that I wrote as an undergraduate philosophy major and remember being excited about at the time, since I finally felt like I understood a bit of what Kant was saying. I am posting it in 2023 after discovering it on my computer. If I recall correctly, the essay drew heavily on the outline of these topics in this book by Jill Buroker.
The roots of Kant’s transcendental idealism can be located in what Kant calls the “Copernican shift” that he intends to enact in the science of metaphysics. On Kant’s view, metaphysicians before him have been wrong about the ‘direction of fit’ between mind and world. That is, metaphysicians—rationalists and empiricists alike—have always tacitly assumed that knowledge depends entirely on the world outside of the perceiver. To obtain knowledge, they assumed, is to discover subject-independent truths about the nature of “things in themselves”. The problem that Kant perceives with this assumption is that it has failed to explain how it can be possible for us to have synthetic a priori knowledge—i.e. necessary and universal knowledge that is genuinely informative about the nature of the world (i.e. that goes “beyond the concepts”). In other words, it fails to show how metaphysics (the domain of which is the synthetic a priori), as a science, is possible (B22).
Leibniz and the rationalists, on the one hand, had occupied themselves with mere conceptual analysis, but had failed to give a coherent account of why their conclusions applied to reality, and had thus failed to do any sort of substantive (synthetic) metaphysics (B22). Hume, on the other hand, had perceived the problem of the synthetic a priori more clearly but had failed to rescue metaphysics, apparently showing (on Kant’s reading) that any sort of informative and necessary knowledge was impossible. Kant agrees with Hume that experience is insufficient to ground a priori knowledge, but thinks that Hume came to the wrong conclusions about synthetic a priori knowledge in general. In particular, Hume overlooked the possibility that knowledge might begin with experience but not arise out of it (B1). That is, Hume overlooked the possibility that the perceiver might make some contribution to the form of experience, if not its content.
This insight—that the subjective mental capacities of the perceiver contribute something to the form of experience—is the Copernican shift alluded to above. For, if such contributions exist, then they must be a priori (i.e. known prior to experience) and can therefore ground the possibility of synthetic a priori judgments. This is the first part of Kant’s transcendental idealism: the view that synthetic a priori knowledge originates in the subjective mental capacities of the knower (i.e. in the forms of thought and sensibility). The consequence of this thesis (and the second part of transcendental idealism) is that synthetic a priori knowledge is only of objects as they appear to us (the phenomenal realm) and not of subject-independent things in themselves (the noumenal realm). Indeed, Kant will hold that we can assert nothing positive about the noumenal realm other than its existence.
With the Copernican shift enacted, Kant’s project is then to find a way to determine what the potential a priori forms of experience might consist in. Importantly, Kant must find a way to make this determination without committing the sin of ‘blind’ metaphysical speculation that he abhorred in the rationalists. Kant’s solution is the transcendental argument: he will begin with incontrovertible facts about experience (thus avoiding mere speculation) and attempt to determine the necessary preconditions of these facts. These preconditions will be the a priori forms of experience. Broadly, Kant undertakes this task in two parts (the ‘transcendental aesthetic’ and the ‘transcendental analytic’) that correspond to the two aspects of experience: intuition and thought. More precisely, in the transcendental aesthetic, he attempts to determine the necessary preconditions of intuition; likewise, in the transcendental analytic, he attempts to determine the necessary preconditions of thought. Ultimately, Kant concludes that spatiality and temporality are the necessary preconditions of intuition, and are therefore its a priori ‘forms’. Likewise, Kant’s table of categories (B106) is his theory regarding the necessary preconditions (i.e. a priori form) of thought.
With these a priori forms of experience outlined, Kant is able to allow for the possibility of metaphysics. For, by reflecting on these a priori forms, we can establish that the world of appearance necessarily conforms to certain fundamental laws. For example, in the second analogy, Kant shows, based on his table of concepts (i.e. the form of thought), that we can determine, necessarily, that every experienced event has a cause. In line with the characterization of transcendental idealism provided above, Kant here does not show that being caused is a necessary feature of things (events) in themselves; rather, he shows that it is a necessary feature of the way events appear to us. This gives a flavor of how Kant’s transcendental idealism allows for the possibility of synthetic a priori of appearances (while denying knowledge of things in themselves).
Kant explicitly contrasts his position with what he calls “transcendental realism”, or the view “that regards space and time as independent of our sensibility and so represents our outer experiences as subject-independent things in themselves” (A369). As we’ve seen, Kant differs from this position because he thinks that time and space are not “independent of our sensibility” but are rather the forms of our inner and outer intuition (i.e. they are transcendentally ideal). Because of this, spatiality and temporality are properties only of appearances and not things in themselves, as the transcendental realist believes. Moreover, it should be clear that, just as Kant is not a realist, he is also not an idealist in anything like the Berkeleyan sense. For, as we have seen, Kant thinks that we have reason to believe that a mind-independent noumenal realm exists (contra the idealist, who denies the possibility of mind-independent substance), though we can say nothing about it.