Idealism is opposed to realism. Kant delineated several different ways in which the idealism/realism opposition had been played out by his predecessors, as well as proposing a new distinction based on his own investigations. He was very defensive about the description of his own philosophy as a version of idealism, given that he was criticized for having "freshened up" the idealism of Berkeley.
Generically, idealism and realism are concerned with the existence of objects in space. A realist affirms positively the existence of objects in space, while an idealist either denies or questions the existence of such objects.
The various forms of idealism and realism are based on two other sets of opposing high-level concepts:
Kant thought that all earlier varieties of idealism were either dogmatic or skeptical. This distinction is one of philosophical methodology. The dogmatic procedure attempts to establish the truth of principles a priori, based on the analysis of philosophical concepts. Dogmatism itself is the attempt to do so without a previous critique of the powers of human reason. Criticism, then, is the "necessary preparation for a thoroughly grounded metaphysics" which itself is subject to the dogmatic procedure (Bxxxv). Skepticism, on the other hand, is a response to the failure of dogmatism, and it denies the possibility of a priori metaphysical principles. Wolff is held up as a paradigmatic practitioner of dogmatism, Hume of skepticism, and Kant himself of criticism.
The dogmatic idealist holds as an a priori principle that objects in space do not exist. In the first edition of the Critique (A377), Kant found the "contradictions" in the concept of matter, to be dealt with, not in the Aesthetic, but in the Second Antinomy. The problem with matter is its infinite divisibility, which conflicts with an a priori demand by reason for a stopping point, an "unconditioned," in the decomposition of things.
According to the second edition of the Critique, the reason for this denial is that the dogmatic idealist holds that space is a property that must belong to things in themselves (B274). Kant agreed that under this assumption, the denial of the existence of objects in space is justified: "For in that case space, and everything to which it serves as condition, is a non-entity" (B274). In the Transcendental Aesthetic, Kant had reflected on "absurdities" following from the assumption. (These absurdities are listed in the page on Newton .) In this connection, Kant explicitly mentioned Berkeley as the paradigmatic dogmatic idealist, apparently on the basis of Berkeley's anti-Newtonian polemic in his work de Motu . In the Prolegomena, Kant associated this kind of idealism with mysticism. If the existence of things in space are denied, the only remaining objects are objects of pure understanding, which could be known (according to Kant) only through mystical or visionary intuition.
Skeptical idealism is sometimes called "problematic." The claim here is that there is no a priori proof of the existence of things in space, the only possible proof being empirical. But no proof from outer experience is forthcoming, since what we take to be experience of things in space may (it seems) be the product of the imagination. And inner experience (it is held), does not provide sufficient evidence for the existence of outer objects (objects existing in space). Kant counted Descartes as a skeptical idealist. This attribution is puzzling, since in the Sixth Meditation, Descartes had attempted to prove the existence of objects in space. Perhaps Kant overlooked this argument, or perhaps he found it to be deficient.
Critical philosophy dispels idealism with respect to the existence of things in space, in that it provides a means of showing a priori that there are spatial objects. Thus it makes possible a form of realism. First, criticism reveals that space is not a property of things in themselves but instead is a form of intuition, to which all appearances must conform. This dispatches dogmatic idealism. Second, criticism provides a new way of proving principles a priori, by showing that they are required for the possibility of experience. In the Refutation of Idealism, Kant argued that inner experience is possible only on the condition of the existence of things in space.
The dogmatic procedure in metaphysics, it was noted above, led to the substantive thesis that space is a property of things in themselves, with the consequence that things in space are things in themselves as well. Since the thesis is thought to follow from the a priori analysis of concepts, it is a transcendental thesis, entitled "transcendental realism." "The transcendental realist . . . interprets outer appearances (their reality being taken as granted) as things in themselves, which exist independently of us and of our sensibility, and which are therefore outside us -- the phrase 'outside us' being interpreted in conformity with pure concepts of understanding" (A369). (These pure concepts are those of the "inner" and "outer.") Correspondingly, transcendental idealism is the thesis (based on criticism) that outer appearances are not things in themselves, and space is not a condition of objects considered as things in themselves.
While the transcendental distinction between idealism and realism concerns the way in which we represent space and the things in it, the empirical distinction is concerned with the evidence for their existence. Empirical realism is the view that experience provides proof of the existence of objects in space, while empirical idealism holds that it does not. Kant pronounced triumphantly that transcendental realism leads to empirical idealism. "After wrongly supposing that objects of the senses, if they are to be external, must have an existence by themselves, and independently of the senses, he finds that, judged from this point of view, all our sensuous representations are inadequate to establish their reality" (A369). Empirical idealism is a skeptical idealism, rather than a dogmatic idealism. This point is obscured by the fact that at A368, Kant pronounced that the term 'idealist' is always meant skeptically.
The other side of Kant's triumphant announcement was the claim that transcendental idealism leads to empirical realism. We do have enough empirical evidence to yield knowledge of the existence of objects in space, though this experience is inner or self-conscious. This claim is argued for in very different ways in the first-edition Fourth Paralogism and the second-edition Refutation of Idealism. In the former, the status of outer objects as representations is the key premise: "the objects are nothing but representations, the immediate perception (consciousness) of which is at the same time sufficient proof of their reality" (A317). In the latter, the claim is that the existence of objects in space makes possible the determination of our inner states in time.
Summary
The issue: how we represent objects in space and whether their existence can be known.
The positions:
Objects in space are things in themselves, a claim based on dogmatic procedure in metaphysics (transcendental realism).
On this assumption, either we know a priori that they do not exist (dogmatic idealism) or we do not know through experience whether they exist (skeptical, problematic, empirical idealism).
Objects in space are appearances, a claim based on critical investigation of the possibility of metaphysics (transcendental, critical idealism).
On this assumption, they exist and we know through experience that they do (empirical realism).
