What we presuppose to know about a thing is not known to us as a thing. This means that the presuppositions of empirical experience are not empirical—they are transcendental. Immanuel Kant’s view of the mind is based on his notion of transcendental apperception, which is not the same as his transcendental idealism.
Apperception is the mind’s capacity to judge according to rule. Without apperception, perception cannot happen. The act of perception runs parallel to the act of apperception. To perceive a thing, the mind must make a judgment based on certain rules—this is the act of apperception. Transcendental apperception is the mind’s ability to tie together all experience. It implies a unity of the self; the self itself appears as a thing that can be perceived as other things outside the self. Transcendental unity of apperception represents the junction at which the perception of the self and the perception of the things undergo a synthesis—the synthesis is made possible by the categories which unite the self and the things that are being perceived. (Kant uses the terms “unity of consciousness” and “unity of apperception” interchangeably and it seems to mean that a man is consciousness of not just one experience but of many experiences.)
Without transcendental unity of apperception, knowledge would be impossible, since we cannot be aware of even the passage of time, an attribute which lies at the root of all experience, and thereby, all knowledge.