In other words: Time is not something present out-there. It is not something that can be empirically intuited. But that means that time in itself is not determinable. I cannot determine any “now” in an absolute way by way of the pure pre-view of the whole of time, because any determinate “now” always already bespeaks a “now, when . . .” Every now-determination is essentially relative to some present thing; and only to the degree that this present thing (with regard to which time is determinable at all) can be fixed, is a determination of time possible.
By the way, Einstein arrived at this same framework for determining time by pursuing some quite specific, concrete problems in physics. The principle of the theory of relativity—that all time is the time of a certain place—is a principle that is grounded in the very essence of time, insofar as what is present in the sense of being present in nature can be determined only place-wise—i.e., only in terms of a place and relative to a place. There is no absolute perception of time. In a certain sense, as regards something present in nature, I can never simply and directly fix its “now” as given absolutely. Instead, the now is always a “now, when . . .”
Time itself as a whole cannot be perceived, i.e., is not empirically intuitable as something present. Nonetheless time does show up as something given, and in such a way that what gives it [352] remains hidden. Then how is a time-determination of time to be possible if every determining of time, every saying of “now,” is a matter of coming back to something present—while that present something is not itself accessible in any absolute way? Kant now says: If time as such is to be determinable, and if in that process time as a whole cannot be comprehended as that to which I come back and from out of which I somehow determine an absolute now-position—then in the appearances themselves one must be able to find antecedently a substratum that presents time. There must be something “that always exists, i.e., something lasting and persisting” (B 225). And what exists in this way is substance. Time is presented—it is rendered sensible—in substance as persistence. As we will show later, time is the schema of substance: time qua persistence presents a rule for determining natural entities as substances. (Substance cannot be intuited any more than time can. Persistence is the rule for rendering something sensible.) The intuition time is the pre-viewed basis-on-which of pure intuition, and as so previewed, time cannot be determined, i.e., cannot be directly comprehended in any synthesis. But to the degree that it is determined, it is determined through a synthesis that, as such, undergirds a rule. Therefore, even though a direct determination of time is intrinsically necessary but essentially impossible, there is a path that we can take via a rule of the understanding—one that, as a rule of the understanding, is antecedently indicated by the “I think” and its original unity.