 Aristotle’s discussion of the fallacy of accident is neither clear nor convincing. One medieval attempt to clarify the fallacy of accident is found in Giles of Rome. The fallacy arises, he said, when there is a variation in the supposition of the middle term. Burley extends the idea of variation of the supposition of terms to the extremes (Burley,  Tractatus super librum Elenchorum, cited in Ockham 1979, p.231 n.3):