Main article: Transcendental idealism
Transcendental idealism, founded by Immanuel Kant in the eighteenth century, maintains that the mind shapes the world we perceive into the form of space-and-time.
… if I remove the thinking subject, the whole material world must at once vanish because it is nothing but a phenomenal appearance in the sensibility of ourselves as a subject, and a manner or species of representation.
— Critique of Pure Reason A383
The 2nd edition (1787) contained a Refutation of Idealism to distinguish his transcendental idealism from Descartes’s Sceptical Idealism and Berkeley’s Dogmatic Idealism. The section Paralogisms of Pure Reason is an implicit critique of Descartes’ idealism.
Kant says that it is not possible to infer the ‘I’ as an object (Descartes’ Cogito ergo sum) purely from “the spontaneity of thought”.
Kant focused on ideas drawn from British philosophers such as Locke, Berkeley and Hume but distinguished his transcendental or critical idealism from previous varieties;
The dictum of all genuine idealists, from the Eleatic school to Bishop Berkeley, is contained in this formula: “All knowledge through the senses and experience is nothing but sheer illusion, and only in the ideas of the pure understanding and reason is there truth.” The principle that throughout dominates and determines my [transcendental] idealism is, on the contrary: “All knowledge of things merely from pure understanding or pure reason is nothing but sheer illusion, and only in experience is there truth.”
— Prolegomena, 374
Kant distinguished things as they appear to an observer and things in themselves, “that is, things considered without regard to whether and how they may be given to us”.[Critique of Pure Reason, A 140]
We cannot approach the noumenon, the “thing in Itself” (German: Ding an sich) without our own mental world.
He added that the mind is not a blank slate, tabula rasa but rather comes equipped with categories for organising our sense impressions.
In the first volume of his Parerga and Paralipomena, Schopenhauer wrote his “Sketch of a History of the Doctrine of the Ideal and the Real”.
He defined the ideal as being mental pictures that constitute subjective knowledge.
The ideal, for him, is what can be attributed to our own minds.
The images in our head are what comprise the ideal.
Schopenhauer emphasized that we are restricted to our own consciousness.
The world that appears is only a representation or mental picture of objects.
We directly and immediately know only representations.
All objects that are external to the mind are known indirectly through the mediation of our mind.
He offered a history of the concept of the “ideal” as “ideational” or “existing in the mind as an image”.
[T]rue philosophy must at all costs be idealistic; indeed, it must be so merely to be honest. For nothing is more certain than that no one ever came out of himself in order to identify himself immediately with things different from him; but everything of which he has certain, sure, and therefore immediate knowledge, lies within his consciousness. Beyond this consciousness, therefore, there can be no immediate certainty … There can never be an existence that is objective absolutely and in itself; such an existence, indeed, is positively inconceivable. For the objective, as such, always and essentially has its existence in the consciousness of a subject; it is therefore the subject’s representation, and consequently is conditioned by the subject, and moreover by thΠηe subject’s forms of representation, which belong to the subject and not to the object.
— The World as Will and Representation, Vol. II, Ch. 1