The overall project of the Deduction is to establish a proof for the necessary connection (and the rules that govern it) between appearances of the outer sense and the faculties of the mind in the inner sense. There needs to be action (wirkung) through synthesis in the mind before “appearances can belong to knowledge or even to our consciousness, and so to ourselves.” This synthesis happens in the ‘imagination‘, a faculty Kant describes as synthesizing the manifolds of the appearances it apprehends. These are all heavily laden terms. Each appearance has a manifold, which must be apprehended before it can be synthesized in the imagination.
The apperception is the unchanging and ultimate foundation for synthesizing the unity of experience. I say “foundation” because Kant says apperception “lies under” all our knowledge of experience. Thus we can be sure that there is no alternative to apperception if there is unity of experience. This foundation must be proved in order for there to be any unity.
The “rules” of the mind are therefore intersubjectively valid, and Kant goes so far as to call these laws. This step-by-step argument is entirely a priori. The concepts are not derived from the objects of perception, not the nature of “self”, and especially not from things in themselves. They are derived from the necessary conditions that precede all empirical knowledge of objects. And to put the final nail in the pyrrhonist’s coffin, these concepts must be unified subjectively in “one and the same apperception”.
However, in the Second Paralogism, Kant had attacked Descartes for using the same principle of the self to establish a dogmatic doctrine of the soul. (See this related entry.) You’ll recall that Kant argued this was an attempt to establish simplicity analytically, which if possible would be false, and if not possible than impossible. In the B Deduction Kant is arguing for the objective simplicity analytically.
This means Kant argues qua the B Deduction what will later be established as an impossible proof or a false proof. If Descartes was “equivocating”, according to Kant, then Kant himself is not only doing the same but is undermining that which the completeness of the entire Critique of Pure Reason rests on, the most important proof of the Critique: the apperception itself.
This foundation, furthermore, is different from the subjective “inner sense”. So if the “I that thinks” can be distinct from the “I that intuits itself”, and yet can be the same subject, why can’t this same simplicity hold for Descartes? All that would need to be done is add some categories, show how they’re necessary, give space and time with manifolds for every appearance, bundle this together and we have knowledge. It would seem that before we read Kant charitably here we should consider why we are not transitively reading into Descartes charitably as well. Kant claims equivocation where Descartes could have made a similar move, yet he does not tidy up Descartes. He distills his views on the self in chapter the Paralogisms of Pure Reason.
The situation we are put in, as charitable readers, is one where our interlocutor has uncharitably interpreted his own interlocutor. And if the principle of charity extends beyond the text at hand, namely to Descartes’s Meditations on First Philosophy, what we have is a philosophical stalemate.