Definitions

The thing in itself: The meaning of this phrase is explained by Kant in his Prolegomena To Any Future Metaphysics: "As the senses... never and in no single instance enable us to know things in themselves, but only their appearances, and as these are mere representations...all bodies, together with the space in which they are, must be held to be nothing but mere representations in us, and exist nowhere else than merely in our thought. Now is this not manifest idealism?"

Kant himself answered this question in the negative. His grounds were that he did not deny that there are things-in-themselves but only that we can know anything of them as they are.

A Dictionary Of Philosophy. Second edition. (Pan Books 1984).