If they adopt the second position (as do some metaphysicians of nature), and hold space and time to be relations of appearances (next to or successive to one another) that are abstracted from experience though confusedly represented in this abstraction, then they must dispute the validity or at least the apodictic certainty of a priori mathematical doctrines in regard to real things (e.g., in space), since this certainty does not occur a posteriori, and on this view the a priori concepts of space and time are only creatures of the imagination, the origin of which must really be sought in experience, out of whose abstracted relations imag ination has made something that, to be sure, contains what is general in them, but that cannot occur without the restrictions that nature has attached to them.