 Heytesbury considers three types of casus: a) possible; b) impossible but imaginable; and c) impossible and unimaginable ([Soph] soph. 18 and 31 [1494: fols. 131va–vb and 162va–vb]). The criteria of possibility, which probably pertain to metaphysics, are not spelled out clearly. The criteria of imaginability are logical and epistemological: