Kant’s epistemology rests on making a fundamental distinction between the thing-in-itself, that is, the thing outside of us given to experience, and the categories of reason under which we experience, and, consequently, know things.
Kant refers to his epistemology as “Transcendental Idealism” because it rejects any notion that we can have knowledge of how the world is in itself, but rather, that we only have the knowledge we impose on the world through our own subjective standpoint, (though as human beings we all have the same standpoint). This standpoint is transcendental insofar that it applies to any experience whatsoever, and is ideal because the way in which we experience the world is constituted by us. It is essential to note, however, that Kant is neither ideal nor subjective insofar that he denies the existence of things outside of us, that is, we do not constitute the things-in-themselves, but only that we constitute the framework of how those things are known and experienced. A ready example of this are the categories of space and time. Kant accepts the Newtonian model of space and time as fixed and universal but denies that either space or time is in the world. Rather, we bring, or “impose”, these categories to the world so that we know things in the spatio-temporal manner we do.
Further, Kant rejects the validity of metaphysics, cosmology, and theology on this basis. He contends that these fields of inquiry arise from the misapplication of the transcendental categories of pure reason. Namely, that instead of applying the categories to empirical inquiry, (read science), we apply the categories to themselves and, as a result, arrive at all sorts of funny ideas about the world and its constitution. As an example of this Kant describes what he calls “antinomies of pure reason”.
I will give one such example of an antinomy: the world. Kant claims that there are two fundamental standpoints concerning the “true” identity of the world: the ideal, such as in Plato, and the empirical, such as in Newton. The ideal claims that the world is the world of intelligibility, or the universals that exist above and beyond the sensible. The empirical claims that the world is the collection of sensible things causally interacting according to the laws of nature.
Kant claims that both standpoints can be argued to satisfactory conclusions, which entails that both suffer from a major flaw––they each have an equally valid antithesis. This leads to the problem of antinomy. Kant problematizes the antinomy of the world as follows:
The problem of idealism arises from taking the categories of reason, space and time, quality and quantity, etc. for the true natures of the world, which the empirical things merely reflect or instantiate. As a result, we can never accommodate the demands of universality in our experience. Our experiences would always fall short. In other words, these universals are too broad for our experience. We would always vainly seek out more and more experiences, which leads to the ultimate rejection of the sensible, empirical, world as true.
The problem of empiricism is as difficult to reconcile. This pertains to the example of Hume’s problem of induction, which led to modern skepticism. In brief, Hume asks how can we have universal knowledge of the laws of nature (Newton’s laws) if the experiences we have of things interacting are necessarily limited? I experience the sun rising and setting everyday but I do not have an infinite amount of experiences to justify that it is following some law. Perhaps Pheobus is doing so at his whimsy. Therefore, our experience is too small to accommodate the laws we attribute to nature. We are not justified in concluding that these laws obtain universally and ultimately, like Hume, reject the validity of universality.
In short, idealism leads to the rejection of the empirical as true and empiricism leads to the rejection of the ideal as true. Kant, as the German he is, seeks a synthesis. He is concerned about truth or true and abiding knowledge. If both can be equally argued how can we choose the right one? In short, how can we arrive at the truth about the world? His solution is that both are wrong because they reject the other. Kant wants to “save the phenomena” as it were. He wants to say that both idealism and empiricism represent are reconciled in one true standpoint.
Kant of course concludes that his transcendental standpoint resolves the issue. It allows us to have universal ideas about the world without attributing that universality to it. Remember, the categories of reason are presupposed by, or ground, our experience. He neither attributes ideal nor empirical experience to the world. In short, we cannot know what the world is per se, but only that we can know the world according to the ground of our experience, which we, not the world, constitute. What follows is that we do not need to “fill” our broad ideals, (as required by Idealism), because the ideals are the basis upon which we experience things in the first place. Secondly, we are not required to have infinite experiences so that true knowledge of natural laws may obtain. Again, in order for us to have any knowledge at all experience must conform to the universal framework we impose. So, things interact causally in space and time and will always do so because we cannot experience them without the categories of space and time, which grounds our experience. Things have universal qualities and unities because, again, these are categories of reason upon which experience is grounded. Both worlds are true because they are grounded in pure reason, in other words, they are one world, the one constituted by us. The world out there, the world of things-in-themselves, we do not know.