  Buridan's account of motion is in keeping with his approach to natural science, which is empirical in the sense that it emphasizes the evidentness of appearances, the reliability of a posteriori modes of reasoning, and the application of naturalistic tropes or models of explanation (such as the concept of impetus) to a variety of phenomena.  He is inclined to dismiss purely theological assumptions as irrelevant to the practice of philosophy, as we see in the following tongue-in-cheek remark: “one might assume that there are many more separate substances than there are celestial spheres and celestial motions, viz., great legions of angels [magnae legiones angelorum], but this cannot be proved by demonstrative arguments originating from sense perception” (QM II.9:  73ra).[53] But there are some theological considerations that must be taken seriously.  He concedes that divine omnipotence is such that it is always possible for God to deceive us in ways we could never detect, although this is tempered by his confidence, for which he cites empirical evidence, that our ordinary powers of perception and inductive inference are sufficiently reliable to make “the comprehension of truth with certitude possible for us” (QM II.1: 9ra). He has little patience for skeptical arguments questioning the possibility of scientific knowledge, such as those he believed to have been advanced by Nicholas of Autrecourt, arguing that it is absurd to demand that all knowledge be demonstrable by reduction to the principle of non-contradiction. Natural science is about what happens for the most part, i.e., assuming the common course of nature (Zupko 2003, chs. 12–13). For the same reason, explanatory models in one special science can be usually applied to others in a way that enhances our understanding of the phenomena. Thus, the concept of impetus recurs in Buridan's psychology to explain the difference between occurrent and dispositional thinking, as well as in his ethics, to account for the relative ease with which virtuous persons are able to do the right thing (they have acquired, or cultivated within themselves, an impetus to act virtuously).