 Given his interpretation of divided modal sentences and consequences like (6:2), Campsall manages to prove the conversion rules. Like Kilwardby, he approximates Aristotle’s original system but in the end does not preserve all of its features. The most interesting features of Campsall’s work, however, are not the result of his efforts to prove Aristotle right, but of his apparently successful solutions. His concept of contingency allows for simultaneous alternatives, such that if something exists, it is possible for it not to exist at that very same moment. Campsall thus abandons the fundamental Aristotelian principle of the necessity of the present (see Knuuttila 1993 and the entry on  medieval theories of modality  for discussion of criticisms of this principle in the late thirteenth century). But Campsall’s analysis is complicated by the fact that, as we have seen, he also accepts the principle that what can exist now does exist now, and that what does not exist now is necessarily non-existent now. In other words, he denies the necessity of the present for affirmative sentences and accepts it for negative ones. There is thus an asymmetry between affirmative and negative modal sentences in Campsall’s system.