 Exactly what al-Fārābī meant by the dictum de omni is hard to make out from Averroes’ account. But al-Fārābī’s use of it seems to contain an attempt to give a set-theoretic justification of  Barbara[16]  in modal contexts. Another quotation from Averroes (Ibn Rushd, Maqālāt 154.18f) shows al-Fārābī using the dictum de omni (again presumably a set-theoretic argument) to justify a syllogism where the major premise has a negative subject: