Prolegomena to Any Future Metaphysics. I see this - the third paragraph:
First, as concerns the sources of metaphysical cognition,
its very concept implies that they cannot be empirical. Its
principles (including not only its maxims but its basic notions)
must never be derived from experience. It must not be physical
but metaphysical knowledge, viz., knowledge lying beyond
experience. It can therefore have for its basis neither external
experience, which is the source of physics proper, nor internal,
which is the basis of empirical psychology. It is therefore a
priori knowledge, coming from pure Understanding and pure Reason.
I see why Ayn Rand metaphorically threw Kant's philosophy across the room. He begins his observation of reality by accepting the bonds of his terminology. It is certainly acceptable to attempt to use the verbal tools available to describe what you find. Metaphorically speaking, great things were done before the advent of modern machinery, but this looks like he's insisting that it is illigitimate to use anything but hand tools.