How is Kant's notion of the Synthetic Unity of Apperception in fact synthetic?

Answer 1

Here is my understanding of it:

Kant claims that each empirically derived sensuous intuition must be accompanied by a "primitive" or "original" spontaneous concept called the "I think." So within a manifold of intuitions derived from a single phenomena, each intuition is accompanied by its own "I think."

In order for these several "I thinks" to be meaningful, there must be a single thinking thing. The SYNTHESIS is the process of fusing these separate "I thinks" into a unity or single consciousness, which Kant called the "Transcendental Unity of Apperception."

So, in Kant jargon, you have a plurality (the "I thinks") fused together or synthesized into a unity (the "I" or self-consciousness) and together, a plurality and unity combine to make a "totality" according to the Quantity category. This totality is what Kant called the "Synthetic Unity of Apperception."