>human knowledge is limited to phenomena, the world as structured by our mind’s categories (space, time, causality, etc)
>the noumenal world, things as they are “in themselves,” independent of perception, is unknowable

this distinction was meant to preserve both empirical science (which studies appearances) and metaphysical limits (beyond which reason cannot go).
but If we truly can’t know anything about the noumenal world, then how can we even assert its existence or claim it causes appearances?