Kant’s assertion that in certain cases the third category is not applicable is misleading. His proof of the validity of the category of reciprocity in the third _Analogy_ really consists in showing that it is necessary to the apprehension of spatial co-existence;[745] and if, as Kant maintains, consciousness of space is necessary to consciousness of time, it is thereby proved to be involved in each and every act of consciousness. It is presupposed in the apprehension even of substantial existence and of causal sequence. His proof that it is a unique category, distinct from the mere combination of the categories of substance and causality, does not, therefore, assume what his words in the above letter would seem to imply, that it is only occasionally employed. The same remark holds in regard to totality; it is presupposed even in the apprehension of a single year. Kant’s references, both here and in other parts of the _Critique_,[746] to totality in its bearing upon the conception of infinitude, reveal considerable lack of clearness as to the relation in which it stands to the Idea of the unconditioned. Sometimes, as in this letter, he would seem to be identifying them; elsewhere this confusion is avoided. In B 111 totality is defined as multiplicity regarded as unity, and in A 142-3 = B 182 its schema is defined as number. (The identification of totality with number has led Kant to say in B 111 that number is not applicable in the representation of the infinite, a much more questionable assertion than that of the letter above quoted.) The statement that necessity is existence in so far as it can be inferred from possibility, or that it is existence given through possibility, is similarly misleading. Kant’s true position is that all three are necessary to the conception of any one of the three. Thus Kant’s reply to Schulze, alike in his letter and in _Note 2_, fails to indicate with any real adequacy the true bearing of Critical teaching in this matter; and consequently fails to reveal the full force of his position. Only in terms of totality can unity and plurality be apprehended; only through the reciprocal relations which determine co-existence can we acquire consciousness of either permanence or sequence; only in terms of necessity can either existence or possibility be defined. The third category is not derived from a prior knowledge of the subordinate categories. It represents in each case a higher complex within which alone the simpler relations defined by the simpler concepts can exist or have meaning.