By Reid's own account, ‘That the natural faculties, by which we distinguish truth from error, are not fallacious’ (FP#7), has a special place among the First Principles of Contingent Truths. Some have found that claim puzzling, but it is not. Contrary to what's usually assumed, certain FPs preceding FP#7 do not already assert the better part of what FP#7 explicitly states. FP#7 is needed because there is nothing epistemological in the FPs that precede it; and its special place among the FPs is a straightforward consequence of its being both perfectly general and distinctively epistemological.
