| Idealism– | Material | Sceptical | Problematic (the position of Descartes). |
| Sceptical in the stricter and more usual sense (the position of Hume). | |||
| Dogmatic (the position of Berkeley). | |||
| Formal or Critical or Transcendental (Kant’s own position). | |||
The distinction between problematic idealism and idealism of the more strictly sceptical type is not clearly drawn by Kant.[1001] Very strangely Kant in this connection never mentions Hume: the reference in B xxxix n. is probably not to Hume but to Jacobi. Transcendental idealism is taken as involving an empirical realism and dualism, and is set in opposition to transcendental realism which is represented as involving empirical idealism. In B xxxix n. Kant speaks of “psychological idealism,” meaning, as it would seem, material or non-Critical idealism.
In the second appendix to the Prolegomena Kant draws a further distinction, in line with that already noted in his lectures on metaphysics. Tabulated it is as follows:
| Idealism– | Mystical, in the sense of belief in and reliance on a supposed human power of intellectual intuition. It is described asidealism in the strict (eigentlich) sense—the position of theEleatics, of Plato and Berkeley. |
Formal or Critical—Kant’s own position. |
This latter classification can cause nothing but confusion. The objections that have to be made against it from Kant’s own critical standpoint are stated below.[1002]
Let us now consider, in the order of their presentation, the various refutations of idealism which Kant has given in his Critical writings.
I. Refutation of Idealism as given in First Edition of “Critique” (A 366-80).—This refutation is mainly directed against Descartes, who is mentioned by name in A 367. Kant, as Vaihinger suggests, was very probably led to recognise Descartes’ position as a species of idealism in the course of a re-study of Descartes before writing the section on the Paralogisms. As already pointed out, this involves the use of the term idealism in a much wider sense than that which was usually given to it in Kant’s own day. In the development of his argument Kant also wavers between two very different definitions of this idealism, as being denial of immediate certainty and as denial of all certainty.[1003] The second interpretation, which would make it apply to Hume rather than to Descartes, is strengthened in the minds of his readers by his further distinction[1004] between dogmatic and sceptical idealism, and the identification of the idealism under consideration with the latter. The title problematic which Kant in the second edition[1005] applies to Descartes’ position suffers from this same ambiguity. As a matter of fact, Kant’s refutation applies equally well to either position. The teaching of Berkeley, which coincides with dogmatic idealism as here defined by Kant, namely, as consisting in the contention that the conception of matter is inherently contradictory, is not dwelt upon, and the appended promise of refutation is not fulfilled.
Descartes’ position is stated as follows: only our own existence and inner states are immediately apprehended by us; all perceptions are modifications of inner sense; and the existence of external objects can therefore be asserted only by an inference from the inner perceptions viewed as effects. In criticism, Kant points out that since an effect may result from more than one cause, this inference to a quite determinate cause, viz. objects as bodies in space, is doubtfully legitimate. The cause of our inner states may lie within and not without us, and even if external, need not consist in spatial objects. Further, leaving aside the question of a possible alternative to the assumption of independent material bodies, the assertion of the existence of such objects would, on Descartes’ view, be merely conjectural. It could never have certainty in any degree equivalent to that possessed by the experiences of inner sense.
“By an idealist, therefore, we must not understand one who denies the existence of outer objects of the senses, but only one who does not admit that their existence is known through immediate perception, and who therefore concludes that we can never, by way of any possible experience, be completely certain of their reality.”[1006]
No sooner is the term idealist thus clearly defined than Kant, in keeping with the confused character of the entire section, proceeds to the assertion (a) that there are idealists of another type, namely, transcendental idealists,[1007] and (b) that the non-transcendental idealists sometimes also adopt a dogmatic position, not merely questioning the immediacy of our knowledge of matter, but asserting it to be inherently contradictory. All this points to the composite origin of the contents of this section.