No determination of things can be intuited before the things exist
No determination of things, absolute or relative, can be intuited prior to the existence of the things to which it belongs, hence not a priori.
The sentence that gives the reason for conclusion (a) at A 26 / B 42 (AA 3:55.06–08). It is stated without argument in that place.
Supports
- a-kant-space-not-of-things-in-themselvesSpace is not a determination of things in themselves, because it is intuited a priori