The active intellect (also translated as agent intellect, active intelligence, active reason, or productive intellect) is a concept in classical and medieval philosophy. The term refers to the formal (morphe) aspect of the intellect (nous), in accordance with the theory of hylomorphism.
The nature of the active intellect was the subject of intense discussion in medieval philosophy, as various Muslim, Jewish and Christian thinkers sought to reconcile their commitment to Aristotle's account of the body and soul to their own theological commitments. At stake in particular was in what way Aristotle's account of an incorporeal soul might contribute to understanding of the nature of eternal life.
The idea is first encountered in Aristotle's De Anima, Book III. Following is the translation of one of those passages (De Anima, Bk. III, ch. 5, 430a10-25) by Joe Sachs, with some notes about the Greek:
The passage tries to explain "how the human intellect passes from its original state, in which it does not think, to a subsequent state, in which it does." He inferred that the energeia/dunamis distinction must also exist in the soul itself. Aristotle says that the passive intellect receives the intelligible forms of things, but that the active intellect is required to make the potential knowledge into actual knowledge, in the same way that light makes potential colors into actual colors.