[yesenadam]

i.e. The Critique of Pure Reason. (Kant wrote 3 Critiques) It's known as one of the most difficult-to-read philosophy books in history, though, so don't expect to pick it up and understand what he's saying. I did an entire university course on the book—I mostly studied philosophy at university—which was gruelling, and sometime later when a girlfriend saw the fat book on my shelf and asked what it's about, looked horrified when I couldn't tell her. Never heard anyone say he was 100% right, though. Particularly his own successors (Neo-Kantians) who, I believe, thought important parts of his system should be dropped—mainly, Kant's reality/the noumenon/thing-in-itself that we can never know or say anything about.

(The most helpful link I can think of is James Franklin's article on Stove's Gem, the "Worst Argument in the World"—Very lucid writing, and not a bad introduction to talk of things-in-themselves. https://web.maths.unsw.edu.au/~jim/worst.html )

[ethn]

Neo-Kantian generally refers to the application of the style and epistemology provided by transcendental idealism in treatment of other subjects, rather than as a departure from Kant.

The Thing-in-Itself was once controversial but undeservedly. It's necessary to prevent the subject knowing the object fully, so thus it is treated in the sense of Cassirer/Cohen's Infinitesimal. No one has trouble with Noumenon maybe only in the disappointment with the intractability of metaphysics to answers some of the most pressing questions.

[ineedasername]

Yes, so difficult to read that he wrote the much more accessible Prolegomena a few years later to summarize his ideas. Critique if Pure Reason is a classic, but what bootstrapped his ideas into the public academic society if the time was Prolegomena.