THIS well-known passage1 practically replaces a long section,2 contained only in the first edition, on the fourth paralogism of pure reason. Its aim is to vindicate against 'idealism' the reality of objects in space, and it is for this reason inserted after the discussion of the second postulate. The interest which it has excited is due to Kant's use of language which at least seems to imply that bodies in space are things in themselves, and therefore that here he really abandons his main thesis.
Idealism is the general name which Kant gives to any view which questions or denies the reality of the physical world; and, as has been pointed out before,3 he repeatedly tries to defend himself against the charge of being an idealist in this general sense. This passage is the expression of his final attempt. Kant begins by distinguishing two forms which idealism can take according as it regards the existence of objects in space as false and impossible, or as doubtful and indemonstrable. His own view, which regards their existence as certain and demonstrable, and which he elsewhere4 calls transcendental idealism, constitutes a third form. The first form is the dogmatic idealism of Berkeley. This view, Kant says, is unavoidable, if space be regarded as a property of things in themselves, and the basis of it has been destroyed in the Aesthetic. The second form is the problematic idealism of Descartes, according to which we are immediately aware only of our own existence, and belief in the existence of bodies in space can be____________________