Transcendental Idealism - Schopenhauer

Schopenhauer

Briefly, Schopenhauer described transcendental idealism as a "distinction between the phenomenon and the thing in itself, and a recognition that only the phenomenon is accessible to us because "we do not know either ourselves or things as they are in themselves, but merely as they appear." Some of Schopenhauer's comments on the definition of the word "transcendental" are as follows:

Transcendental is the philosophy that makes us aware of the fact that the first and essential laws of this world that are presented to us are rooted in our brain and are therefore known a priori. It is called transcendental because it goes beyond the whole given phantasmagoria to the origin thereof. Therefore, as I have said, only the Critique of Pure Reason and generally the critical (that is to say, Kantian) philosophy are transcendental.

— Parerga and Paralipomena, Volume I, "Fragments for the History of Philosophy," § 13

Schopenhauer contrasted Kant's transcendental critical philosophy with Leibniz's dogmatic philosophy.

With Kant the critical philosophy appeared as the opponent of this entire method . It makes its problem just those eternal truths (principle of contradiction, principle of sufficient reason) that serve as the foundation of every such dogmatic structure, investigates their origin, and then finds this to be in man's head. Here they spring from the forms properly belonging to it, which it carries in itself for the purpose of perceiving and apprehending the objective world. Thus here in the brain is the quarry furnishing the material for that proud, dogmatic structure. Now because the critical philosophy, in order to reach this result, had to go beyond the eternal truths, on which all the previous dogmatism was based, so as to make these truths themselves the subject of investigation, it became transcendental philosophy. From this it follows also that the objective world as we know it does not belong to the true being of things-in-themselves, but is its mere phenomenon, conditioned by those very forms that lie a priori in the human intellect (i.e., the brain); hence the world cannot contain anything but phenomena.

— The World as Will and Representation, Vol. I, Appendix: "Criticism of the Kantian Philosophy"