 Perhaps the most striking feature of Avicenna’s system is the productivity (and foundational role) of first-figure syllogisms with a possibility minor premise, which has traditionally been regarded as problematic by many of Avicenna’s commentators.  The case in support of this view hinges primarily on a certain reading of the terms and the idea that possibility propositions express relations of compatibility (or separability) among conceptions or essences, which Avicenna often spells out in terms of natures (of the subject and predicate) (Thom 2008, 2012).