Can you quote Kant to demonstrate that distinction? — David Mo
I already did!: "Understanding accordingly limits sensibility, but does not thereby extend its own sphere. In the process of warning the latter that it must not presume to claim applicability to things-in-themselves but only to appearances". (A288)
That is: the limitation set by the understanding on sensibility
does not apply to things-in-themselves. Noumena are 'appearence-relative', and
only appearence-relative. Things-in-themselves are not.
Further down: "If we want to call this object a noumenon because the representation of it is nothing sensible, we are free to do so. But since we cannot apply any of our concepts of the understanding to it, this representation still remains empty for us, and serves for
nothing but to designate the boundaries of our sensible cognition" (B346, my emphasis)
It should be noted to that your partial quote (B346), in context, comes at the end of a discussion in which the idea that limitations of understanding extends beyond sensibility, is a
mistake. So the start of Kant's sentence, which you conveniently cut off, reads:
"We therefore think something in general, and on the one side determine it sensibly, only we also distinguish the object represented in general and
in abstracto from this way of intuiting it; thus there remains to us a way of determining it merely through thinking that is, to be sure, a merely logical form without content, but that nevertheless
seems to us to be a way in which the object exists in itself (
noumenon), without regard to the intuition to which our sensibility is limited".
But this 'seems' is precisely, a mistake. To regard the noumenon as that 'without regard to which the intuition to which our sensibility is limited' is an error: the whole section is a 'critique of pure understanding': a critique which posits that to think noumena as abstracted from sensibility is a total mistake. What you quote in defence of your position is for Kant paradigmatic of a transcendental exercise of the understanding which must be avoided at all costs!