What we are reading, the Prolegomena, is a restatement of some of the main ideas of Kant’s main work, the Critique of Pure Reason. He wrote it in order to correct what he saw as misreadings of the Critique. The aim of reading it is to get an idea of what Kant was trying to say. The full arguments, I’m afraid, are in the longer work.
The question Kant seeks to answer is “whether such as thing as metaphysics is possible at all?” (Kant [1783] 2019, 255/719)
What that means is, can we demonstrate anything about metaphysics? That is, the nature of reality. Is it material or at least partly immaterial? Do we have free will? What are the conditions of a thing remaining the same over time? Why is there a necessary connection between cause and effect? And so on.
Why ask such a question? Kant thought that Hume had effectively thrown the whole enterprise into doubt. In particular, he was impressed by Hume’s arguments that we cannot know a priori that causes make their effects necessary (Kant [1783] 2019, 257–58/720-21). As Kant famously put it, reading Hume interrupted him from his “dogmatic slumbers” (Kant [1783] 2019, 260/721).
Kant’s answer is that the natural experience that Hume thought was our only source of knowledge about the world (such as it is) must be structured by reason. Critical concepts such as space, time, cause, and substance are not derived from experience but are the “pure forms of thought” that make experience possible.
He calls this “transcendental idealism.” It is transcendental because it involves showing that something that we know we have, namely, experience, depends on something else, namely the pure forms of thought. It is idealism because the preconditions of our experiences are ideas rather than being found outside our minds.
Nonetheless, Kant insisted that he was not an idealist like Berkeley. He does not, he says, think that there are no objects outside of the mind (Kant [1783] 2019, 293/737). Rather, he thinks that our thoughts about those objects are necessarily structured by other thoughts. It is fair to ask whether that is a significant difference or not.
Kant sets up his inquiry with another question: how are synthetic a priori propositions possible (Kant [1783] 2019, 276/728)?
Analytic judgments are uninformative: the subject contains all the information that is included in the rest of the judgment. “A bachelor is an unmarried man,” for example, is just the definition of a bachelor.
Synthetic judgments are informative. “Philosophy 42 is hilarious” is something you have to investigate; the concept of philosophy 42 alone does not tell you whether it is, in fact, hilarious or not.
Our knowledge based on the senses is synthetic. Kant claims that some synthetic judgments are based on reason rather than the senses. So the question about synthetic a priori propositions is whether any of them can be informative rather than repeating definitions.
We discussed two examples meant to show that there are, in fact, synthetic a priori judgments. Next time we will talk about another example: causation.
One involves arithmetic. Kant believes that a simple equation like 7 + 5 = 12 is synthetic rather than analytic. The concepts of “5,” “+,” “7,” and “=” do not themselves determine the answer. I have to confess that I am not sure exactly why he believes this. Hume thought that it was what he called “demonstrative” and based on a comparison of ideas, which sounds a lot like what Kant meant by “analytic.”
A second example involves geometry. Kant maintains that the concept of a triangle does not tell you whether two triangles with equal angles and sides are congruent, meaning the one can be perfectly placed on top of the other. You need to know whether the triangles are in a flat plane or on a sphere. If they are on a plane, you can rotate them on their shared side and they will match. If they are on a sphere, the one that is rotated will not lay flat but will rather stick up.
So what? Well, Kant thinks this shows that we need what he calls a “pure form of sensibility,” namely, space, in addition to our concepts in order to know whether the two triangles are congruent or not. He maintains that the intuition of space must be prior to experience rather than being derived from experience.
Again, I am not confident that I understand why he thinks this is so. There are plenty of empiricist accounts of how we get our ideas of space (and time) from experience. Kant thinks they are wrong, but I do not see that we saw his reasons here.