ways of unifying—a “Table of Judgments”—as the possible forms of combining. He thinks that this Table provides the totality of possible forms of unity, the possible forms of pre-viewing for these specific ways of combining. These unities, conceptually grasped as the various possible pre-viewed bases-on-which of combining, are the categories. With that, the transcendental, a priori structure of the second stem of knowledge—the understanding—is laid out. The structure of the first stem—sensibility—was delineated as the forms of intuition, space and time.
Givenness as such is possible only in a “for” that is constituted by an original synthesis that is expressed as the “I think.” This synthesis is the condition of the possibility of every concrete act of combining; and (a priori) transcendentally it is the condition of the possibility of the primary function of unity, namely, judging as a pure action of the understanding. These pure concepts of unity, which pertain to all its ways of functioning, are supposed to be endowed with a priori content. (They are, after all, determinations of the content of nature’s being.) Where does this a priori content come from since, as transcendental, it cannot be drawn from experience? How can these pure concepts of understanding, as unities constitutive of merely empty actions of understanding, have any relation to objects [Objekte], to content-determined objects [Gegenstände]? What is given as essentially a priori, and what is given universally?—specifically, given in such a way that (1) it is something given in general for every action of the understanding that is supposed to determine something in the object [Objekt]; and so that (2) this universal given determines every empirical given in its being-given?
This a priori given, which a priori and universally lies “in front of” the understanding and which, at the same time, determines everything that can be given to sensibility, is, according to Kant, time. Because it is a form of the givenness  of inner sense, it is a form of the givenness of that to which an action of the understanding, as an action of the subject, can first and only direct itself. (The understanding remains “in the subject.”) But then it is not a form of outer sense, much less a form of those appearances that natural knowledge is supposed to determine. Therefore we must first show how time, which is primarily and properly the form of inner sense, can also be the form of outer sense and its givenness, and consequently has to be the a priori to which, first of all and without exception, every action of the understanding, as a combining of the given, must be referred.
§27. Time as the universal a priori form of all appearances
To what degree is time, as pure intuition, the universal form of all givens? How does Kant show that time, although primarily and properly the form only of inner sense, is also the form of outer sense?