 As Tobias Rosefeldt (2022) points out, a longstanding puzzle for the B160–1 note is that it references a notion of synthesis that precedes all concepts, while the notion of synthesis to which Kant appeals in the preceding sections of the B-Deduction is one that does employ concepts, i.e., the categories, and thus does not precede concepts. Commentators generally agree that this synthesis that precedes all concepts must be subject to the unity of apperception (e.g., Messina 2014, Williams 2018), but how might it be more thoroughly characterized? Rosefeldt proposes an account of the second step that at this point features the notion of a decomposing synthesis, which contrasts with a composing synthesis. Kant, as he points out, introduces the notion of a decomposing synthesis only in the Second Antinomy (A505/B533, A524/B552), but in Rosefeldt’s view this notion is aptly suited for Kant’s objectives for the second step. While a compositional synthesis constructs a whole from given parts,a decomposing synthesis begins with a given whole and then decomposes it in some way into parts. In this context, it decomposes space as we are intuitively aware of it into one or more of its finite parts, such as spheres or cubes, while the remaining infinite spatial region excluded by those parts is represented as their phenomenal background or horizon (cf., Husserl 1913, 1939). 