After characterizing the mind (nous) and its activities in De Anima iii 4, Aristotle takes a surprising turn. In De Anima iii 5, he introduces an obscure and hotly disputed subject: the active mind or active intellect (nous poiêtikos). Controversy surrounds almost every aspect of De Anima iii 5, not least because in it Aristotle characterizes the active mind—a topic mentioned nowhere else in his entire corpus—as ‘separate and unaffected and unmixed, being in its essence actuality’ (chôristos kai apathês kai amigês, tê(i) ousia(i) energeia; DA iii 5, 430a17–18) and then also as ‘deathless and everlasting’ (athanaton kai aidion; DA iii 5, 430a23). This comes as no small surprise to readers of De Anima, because Aristotle had earlier in the same work treated the mind (nous) as but one faculty (dunamis) of the soul (psuchê), and he had contended that the soul as a whole is not separable from the body (DA ii 1, 413a3–5).

While not strictly a contradiction, this constellation of views does present a serious interpretative difficulty. How could the active mind be separable if it is a capacity of the soul and the soul is not separable? How, indeed, might a capacity be separated from that of which it is a capacity? Typically we expect capacities to be grounded and not free-floating. In what precise sense, then, is the active mind separable, conceptually or ontologically? For that matter, from what precisely is the active mind held to be separable—the body, the other faculties of the soul, or some unspecified category of being? What, exactly, is the active mind (nous poiêtikos) and how is it related to the unqualified mind (nous) discussed in the previous chapter? Should we, in fact, even regard the active mind as a capacity of the human soul, or is that merely an assumption of some of Aristotle’s readers? If it is indeed a capacity of the human soul, does this imply that