But the intellectual soul does not lie open to receive impressions of the likenesses of things that are in phantasms in the way that the likeness exists in the phantasm, but according as those likenesses are raised to a higher stage, by being abstracted from individualising material conditions and rendered actual objects, or terms, of understanding. And therefore the action of the active intellect upon the phantasms precedes their being received into the potential intellect; and thus the prime agency is not attributable to the phantasms, but to the active intellect.
There are some animals that see better by night than by day, because they have weak eyes, which are stimulated by a little light, but dazzled by much. And the case is similar with our understanding, which is "to the clearest truths as the bat's eye to the sun" (Aristotle, Metaph. I, Appendix): hence the little intellectual light that is connatural to us is sufficient for us to understand with. But that the intellectual light connatural to our soul is sufficient to produce the action of the active intellect, will be clear to any one who considers the necessity for positing such an intellect. Our soul is found to be in potentiality to intelligible objects as sense to sensible objects: for as we are not always having sensations, so we are not always understanding.* These intelligible objects Plato assumed to exist by themselves, calling them 'Ideas': hence it was not necessary for him to posit any 'active intellect' rendering objects intelligible.* But if this Platonic position were true, the absolutely better objects of intelligence should be better also relatively to us, and be better understood by us, which is manifestly not the case: for things are more intelligible to us which are nigher to sense, though in themselves they are less excellent objects of understanding. Hence Aristotle was moved to lay down the doctrine, that the things which are intelligible to us are not any self-existent objects of understanding, but are gathered from objects of sense. Hence he had to posit some faculty to do this work of making terms of understanding: that faculty is the active intellect. The active intellect therefore is posited to make terms of understanding proportionate to our capacity. Such work does not transcend the measure of intellectual light connatural to us. Hence there is no difficulty in attributing the action of the active intellect to the native light of our soul, especially as Aristotle compares the active intellect to light (De anima, III, v, 2).
2.76 : That the Active Intellect is not a separately Subsisting Intelligence, but a Faculty of the Soul
2.78 : That it was not the Opinion of Aristotle that the Active Intellect is a separately Subsistent Intelligence, but rather that it is a Part of the Soul