 In [Syllogism] and [Short Syllogism] Al-Fārābī recognizes three figures of categorical syllogisms, following  Aristotle.[13]  In these three figures he recognizes respectively four, four, and six valid moods, exactly as listed by Smith (2018). Following Aristotle again, he regards the first figure syllogisms as “perfect”, i.e., self-evident and not in need of justification. For the moods in second and third figures he gives justifications by reduction to first figure, following Aristotle in the main. (His chief departure from Aristotle in these justifications is that he never uses proof by reductio ad absurdum, though he does recognize such proofs as valid.) Aristotle’s justification of  Darapti[14]  relies on the conversion implication: from “Every B is an A” infer “Some A is a B” (cf. [Syllogism] 30.3). This implication fails to hold if we allow “Every B is an A” to be true when there are no Bs. Accordingly al-Fārābī declares that “Every B is an A” is false unless there is at least one B ([Categories] 124.14). Though he sometimes seems to forget he said this, he is the earliest logician on record as having spelled out this implication of “Every B is an A”.