The first proposition of Schopenhauer's philosophical system is, 'The world is my presentation.' By this he means that it presents itself as appearance to the knowing subject. This appearance is in the forms of time, space and causality. Under these forms every phenomenon necessarily appears, because they are _a priori_ forms of the subject. The world as it presents itself consists entirely of phenomena, that is, appearances, related according to these forms. The most fundamental form of all is the relation between object and subject, which is implied in all of them. Without a subject there can be no presented object. Schopenhauer is therefore an idealist in the sense in which we call Berkeley's theory of the external world idealism; though the expressions used are to some extent different. The difference proceeds from his following of Kant. His Kantianism consists in the recognition of _a priori_ forms by which the subject constructs for itself an 'objective' world of appearances. With Berkeley he agrees as against Kant in not admitting any residue whatever, in the object as such, that is not wholly appearance. But while he allows that Berkeley, as regards the general formulation of idealism, was more consistent than Kant, he finds him, in working out the principle, altogether inadequate. For the modern mind there is henceforth no way in philosophy except through Kant, from whom dates the revolution by which scholastic dualism was finally overthrown. Kant's systematic construction, however, he in effect reduces to very little. His is a much simplified 'Apriorism.' While accepting the 'forms of sensible intuition,' that is, time and space, just as Kant sets them forth, he clears away nearly all the superimposed mechanism. Kant's 'Transcendental AEsthetic,' he says, was a real discovery in metaphysics; but on the basis of this he for the most part only gave free play to his architectonic impulse. Of the twelve 'categories of the understanding,' which he professed to derive from the logical forms of judgment, all except causality are mere 'blind windows.' This alone, therefore, Schopenhauer adopts; placing it, however, not at a higher level but side by side with time and space, Kant's forms of intuition. These three forms, according to Schopenhauer, make up the understanding of men and animals. 'All intuition is intellectual.' It is not first mere appearance related in space and time, and waiting for understanding to organise it; but, in animals as in man, it is put in order at once under the three forms that suffice to explain the knowledge all have of the phenomenal world.