   The divergent sources present in Al-Kindî make their appearance again in his short treatise On  Intellect.[8] Aristotle's Hellenistic commentators had long since elaborated on Book Three of his De anima, and discerned stages of cognition that the master had left too vague for their liking. In al-Kindî's case, John Philoponus appears to have provided a model for al-Kindî's scheme of four kinds of intellect (Jolivet, pp. 50–73; Endress, 197). The first intellect is that which is in act always, a separate principle of intelligibility that contains the species and genera, i.e., the universals, of our world. This first intellect, like that of Alexander of  Aphrodisias,[9] endows our potential intellects (the second intellect) with the abstract ideas they are capable of receiving; which ideas become part of a (third) acquired but passive intellect. When active, however, the (fourth) intellect emerges as just that, an intellect in act, in which its subject is united with its object. As such, the first and fourth intellects are alike phenomenologically, though different quantitatively.