of the I is something that is [a priori] “related-to”—that is what makes up the original unity of the cogito. This is the original unity present in the synthesis of the manifold as such with the I that exists insofar as it is a cogitatio, i.e., a priori—but a unity as pure apperception: a “transcendental unity.” [325]
When we read Kant’s explanation of the pure apperception, we see clearly how much he struggles to make this issue understandable: this ultimate structure of the ur-action of understanding qua synthesis. Nothing might seem be more obvious than that consciousness of something is at the same time self-consciousness. It might seem that nothing more could be made of it. But for Kant a further question arises: What is the ground of the belonging-to-me-ness of the given?
I can only place the manifold presentations next to one other, and only further unify the unifications themselves (i.e., present an ensemble of the manifold of presentations) in such a way that, in doing so, I comprehend myself in each case as the same combining I. But [this is possible] only on the assumption that there is already given beforehand the possible togetherness of the given manifold as such as a whole with the I that thinks a determined manifold. This is the “ground of the identity of apperception” (B 134). This togetherness of the given as such with the I is already given beforehand in a prior synthesis in which this original “unity [is] antecedently thought,” “thought beforehand” (B 133–134, note).
And thus the synthetic unity of apperception is the highest point to which one must affix all use of the understanding, even the whole of logic and, after it, transcendental philosophy; indeed this faculty is the understand-ing itself. (B 134, note)
Therefore, it is the understanding as combining that constitutes the most original a priori, as we discovered in our general reflection at the beginning of this treatment, when we called this argumentation “extrinsic.” How is Kant’s explanation different from that?
Kant says that combination entails unity, a unity that is already presupposed as object of a pre-view. But in the analysis that we just finished, we saw the exact opposite: It is the original synthesis that makes up the unity.
The original synthetic unity of self consciousness is: [326] (1) the originary synthesis which makes unity possible, and (2) the originary unity which makes synthesis possible. The second is constitutively contained in the first. Synthesis entails the self-positing-in-its-identity on the part of the I, in fact of the I that thinks, that presents something. As a priori, unity is a consistent, ever-present character of the cogitatio. It is the constant self-identity of combining in every act of combining.