 In Chapters 3 and 8–22 of Book I of the Prior Analytics, Aristotle extends his theory to include syllogisms with modally qualified categorical sentences. An Aristotelian modal syllogism is a syllogism that has at least one premise modalized, i.e., that in addition to the standard terms also contains the modal words ‘necessarily’, ‘possibly’ or ‘contingently’. Aristotle’s terminology is not entirely clear, however. He speaks only of necessity and possibility, though he works with two notions of possibility. In what seems to be his preferred sense, used primarily in the Prior Analytics, possibility is defined as that which is not necessary and not impossible. This sense of possibility was called contingency in the Middle Ages. But there is another sense of possibility in Aristotle’s On Interpretation according to which possibility is equivalent to what is not impossible. The first concept of possibility, which I will henceforth call ‘contingency’, is used in the modal syllogistic. The second concept is not treated systematically in the Prior Analytics.