The key here is that even as a realist, Aristotle allowed his ideas to trump observation. The reason is simply that he shared a worldview with his contemporaries that saw logic and ideas as the key to truth. This fundamental belief shaped his thought so that he did not see that his epistemology was disconnected from his metaphysics, or, to put it another way, that his approach to knowledge about the physical world was inconsistent with his belief in the priority of particulars over universals and thus of observation over logic.
