1.) The basic data or problem. You go to algebra class and you learn what is essentially Cartesian algebra; you go to physics class an learn classical or Newtonian physics (it will later develop into something else, but only after you learn the Newtonian sort first); you go to a geometry class and you learn a more or less Euclidian geometry as adapted through Descartes. But philosophy is not like this: you come to a philosophy class and you learn philosophies. Why is there no broad consensus on method? Why haven’t we decided on one philosophy and just gone with it? Why is it that when we raise the fundamental questions about God, the universe, the human self that we think we can’t teach one thing but only many? Notice that this is in spite of philosophy being much older and much more broadly practiced than many sorts of math and all sorts of sciences. Isn’t this a scandal?
2.) The Kantian solution. Kant’s solution is simple: you can know things like math and science but not philosophy, or at least “philosophy” so far as it deals with the fundamental questions of God, the universe, and the soul, i.e. metaphysics.
3.) Some jargon. I think that Kant’s fundamental insights are startlingly simple and even self-evident, but they can be obscured by his jargon. We’ll start with one key term: pure. The pure is a cognitive power considered without an object. So an eyeball is “pure vision”; an ear is “pure hearing”. They’re just a structures for seeing and hearing, and considered just as structures we don’t think of them as seeing or hearing, just as divided up into parts (think of a picture of an eye on an optometrists wall. That’s “pure vision”.)
We’ve already seen the a priori and the a posteriori as divided into the necessary and the changeable. We’ll now add the idea of the analytic and the synthetic. Synthetic knowledge is more or less new insights, analytic is just an explication of what one already means. Saying that water is a liquid or wet or clear are all analytical claims, since they are simply ways of making clear what one means when he’s talking about water. Saying that water is H2O is a synthetic statement, since it involves something more than just what one means by “water”. The line between the synthetic and the analytic might not always be clear, and it might vary slightly on our experience, but the basic distinction is sound.
Now in order to have science, or at least the sort of science that would get past the problem we spoke of in (1), it would have to be both a priori – more or less necessary; and synthetic, that is, involving a deepening insight beyond or initial or first experience of a thing and what we first mean by it. This is why Kant sees the basic problem of the search for knowledge as trying to figure out whether synthetic a priori knowledge is possible.
4.) A definition of knowing. If this is right, then an actual piece of knowledge is just the combination of “pure reason” and an object in the world. The world pours into the eye, and the eye picks up on some information and not on others. But we can distinguish different aspects of the world and pure vision. The world seems to explain how there can be any content to our knowledge at all, but it is the eye that explains why we see this color as opposed to another. If we were deer, then we wouldn’t see bright orange and a different color from a tree; if we were a bee we’d see two colors on a sunflower petal and not just a single uniform yellow. If we were dung beetles, we’d experience the sweet scent of dung. This is even true of things like shape – that we see things as curved or straight or three dimensional or two dimensional is due to the structure of the eye. We saw these sorts of arguments in Berkeley, though they can also be seen as applications of the Scholastic principle that all that is known is known according to the mode of the knower.
5.) The fundamental distinction in something known. All knowledge gets its content from the world and its structure from pure cognition. Both are completely unintelligible in themselves. To be more precise, we can know that the world and pure reason exist but not what they are in themselves. Without the world, mind would have no content at all to detect or respond to; without the pure reason, the world would have no determinate features.
6.) The fundamental determinations. The fundamental determination in experience appear to be space and time. The combination of the world and pure reason takes time and happens somewhere. The various divisions in this “somewhere” give us geometry; the various numerical divisions we make in time (like on a clock) give us numbers and hence arithmetic. The possible determinations of this field are not given from the beginning, and so knowledge in this area can be really synthetic. At the same time, since they are fundamental to all thought, they can be a priori. And so math can be a veritable science.
It makes sense that so far s we understand nature formally through math, then nature too can take part in synthetic a priori knowledge. But this is the basic Newtonian program, which was not fundamentally altered even by Einstein. And so the basic Newtonian program of physics is also synthetic a priori knowledge.
7.) The fate of metaphysics. We’ve already seen that what we know is not the world nor pure reason, but only the product made from them. We can know that there is a world but nothing about it, whether it is one or many, material or non material, finite or infinite in time, etc. This includes a fortiori whether it is caused or uncaused, and this rules out any inference to God from nature. Said another way, metaphysics wanted to be a study of how the world is in itself, but this is “the world” that is never known in itself, but only as the purely undetermined content for pure reason.
Just as the world is not known, so also the self isn’t. We can’t know if we are soul or body, or some mix of the two. Self and world are two limits that are never given in themselves. We can no more argue to God from the self, as Descartes did, than from the world.
8.) An important label. The fact that the world is only known so far as it receives determinations from the pure cognitive power is called The Copernican turn.
9.) A line of critique. Any Critique of Kant must happen on a very fundamental level. St. Thomas would probably deny the claim that knowledge is a combination of pure reason and the world. This is fine as a description of sense knowledge, he would say, but intellectual knowledge has no determinate structure. This is exactly what STA means when he says that the intellect is immaterial or that it cannot have any determinate form since this would impede its knowledge of being. If this is right, the dispute between Kant and Scholastic philosophy will trace back to the question whether the intellect has a determinate structure. It seems clear that if it really knows being or substance that, in fact, it can have none. But it is only in Kant that this dispute becomes really clear.