On the Immortality of the Soul

[...the] agent is either God or propositions, since Aristotle declared the intellect of man to be divine and impassible, and to be the most excellent part of the rational soul; from whose opinion [Themistius] subsequently gathers that Aristotle thought the active intellect was either something of ours or plainly ourselves. Thus far Themistius, and, as it seems to me, he most clearly asserted that the active intellect is both in the soul itself and is immortal. Then indeed, he wonders concerning the intellect itself, whether it be one or many, and he withdraws to the position that there is one who illuminates. Yet many are illuminated, and are subsequently illuminating, like a single sun and light distributed and divided among many gazes. For this reason, [he says] that Aristotle compared the intellect to light, and Plato to the sun: because Plato wished the intellect and the soul of man to be as like unto God as possible. Nor does he teach that it should seem absurd if all men, who are said to be composed of act and potency, are referred to a single active intellect: on account of common preconceptions, on account of mutual understanding, and on account of instruction and doctrine.

These words have been an occasion of error for many who did not consider what he had previously discussed concerning the intellect, and what he discusses afterward while he decrees the intellect of power—according to the words and opinion of Aristotle—to be impassible, immortal, and separate; and likewise that which is the agent is separate, and indeed more so, as if someone, he says, were to say that the sun is more separate than its light. However, he asserted that the passive intellect is individual to the body, inseparable, and perishable—that is, the sensory part of the soul, in which are boldness, fear, and hope—and because certain philosophers did not weigh this, they wandered from the path. Finally, he cites Theophrastus teaching that the intellect of power is just as separate from the body and impassible as the agent is separate; thus it is both innate and coming from without—which he warns must not be so established as to be considered truly applied or imported from the outside, but as that which is accustomed to grasp and embrace us immediately from birth. He does not in any way deny the passive intellect of power, but [conceives it] so that it may arrive at act and perfection; and when he had discussed more things from the opinion of Aristotle and Theophrastus, he concludes that the passive and corruptible intellect (which they call "common" and not separate from the body) is one thing, and quite another is that whole which is composed of the active intellect and the intellect of power