  Aristotle develops his modal syllogistic in Prior Analytics 1.8–22. He settles on two-sided possibility (contingency) and tests for syllogismhood all possible combinations of premise pairs of sentences with necessity (N), contingency (C) or no (U) modal operator: NN, CC, NU/UN, CU/UC and NC/CN. Syllogisms with the last three types of premise combinations are called mixed modal syllogisms. Apart from the NN category, which mirrors unmodalized syllogisms, all categories contain dubious cases. For instance, Aristotle accepts: