(which is given by intuition), that of the act of synthesis, and that of the unity; this last is two-fold, an objective unity (the conception of an object in general in which the manifold is united), and a subjective unity (the unity of consciousness under which or, rather, through which the connection is effected).  The categories represent the different kinds of combination, each one of these, again, being completed in three stages, which are termed the Synthesis of Apprehension in Intuition, the Synthesis of Reproduction in Imagination, and the Synthesis of Recognition in Concepts.  If I wish to think the time from one noon to the next, I must (1) grasp (apprehend) the manifold representations (portions of time) in succession; (2) retain or renew (reproduce) in thought those which have preceded in passing to those which follow; (3) be conscious that that which is now thought is the same with that thought before, or know again (recognize) the reproduced representation as the one previously experienced.  If the mind did not exercise such synthetic activity the manifold of representation would not constitute a whole, would lack the unity which consciousness alone can impart to it.  Without this one consciousness, concepts and knowledge of objects would be wholly impossible.  The unity of pure self-consciousness or of “transcendental apperception” is the postulate of all use of the understanding.  In the flux of internal phenomena there is no constant or abiding self, but the unchangeable consciousness here demanded is a precedent condition of all experience, and gives to phenomena a connection according to laws which determine an object for intuition, i.e., the conception of something in which they are necessarily connected.[1] Reference to an object is nothing other than the necessary unity of consciousness.  The connective activity of the understanding, and with it experience, is possible only through “the synthetic unity of pure apperception,” the “I think,” which must be able to accompany all my representations, and through which they first become mine.

[Footnote 1:  Object is “that which opposes the random or arbitrary determination of our cognitions,” and which causes “them to be determined in a certain way a priori.”]

Experience (in the strict sense) is distinguished from perception (experience in the wide sense) by its objectivity or universal validity.  A judgment of perception (the sun shines upon the stone and the stone becomes warm) is only subjectively valid; while, on the other hand, a judgment of experience (the sun warms the stone) aims to be valid not only for me and my present condition, but always, for me and for everyone else.  If the former is to become the latter, an a priori concept must be added to the perception (in the above case, the concept of cause), under which the perception is subsumed.  The category determines the perceptions in view of the form of the judgment, gives to the judgment its reference to an object, and thus gives to the percepts, or rather, concepts (sunshine and warmth), necessary and universally valid connection.  The “reason why the judgments of others” must “agree with mine” is “the unity of the object to which they all relate, with which they agree, and hence must also all agree with one another.”