In several texts, Thomas Aquinas employs a controversial demonstration for the spirituality (that is, intrinsic independence from matter) of the human potential intellect—an argument deriving from a famous, though somewhat abstruse, passage in Book III of Aristotle's De Anima. Because Aquinas's development of this intriguing proof can itself be accused of insufficient rigor, rendering it susceptible to objections, it behooves us to explore this problem from the beginning. Hence, at the very outset, I reproduce two standard translations of the controverted passage from Aristotle's treatise on psychology:
Therefore, since everything is a possible object of thought, mind, in order, as Anaxagoras says, to dominate, that is, to know, must be pure from all admixture; for the co-presence of what is alien to its nature is a hindrance and a block: it follows that it too, like the sensitive part, can have no nature of its own, other than that of having a certain capacity. Thus that in the soul which is called mind (by mind I mean that whereby the soul thinks and judges) is, before it thinks, not actually any real thing. For this reason it cannot reasonably be regarded as blended with the body: if so, it would acquire some quality, e.g. warmth or cold, or even have an organ like the sensitive faculty: as it is, it has none. It was a good idea to call the soul “the place of forms,” though (1) this description holds only of the intellective soul, and (2) even this is the forms only potentially, not actually.
And, since the intellect [can] think every [object of thought], it must exist without being blended [with something else] in order that, as Anaxagoras says, “it may rule,” that is, in order that it may know. For, if it appears along [with some other thing], the [latter will] prevent or obstruct [the knowledge of] another kind [of thing]; hence it is necessary for [the intellect] to be of no nature other than that of [mere] potentiality. So the part of the soul which is called “intellect” (by “intellect” I mean that [part] by which the soul [can] think and believe) is actually none of the things prior to thinking. In view of this, it is not even reasonable that it should be blended with the body, for it might then acquire some quality, e.g., coldness or heat, or there might be even an organ [for it], as there is for the sentient power; but, as it is, there is no [such organ]. So those who say that the soul is a place of forms speak well, except that it is not the whole soul but only the thinking part of it, and that [that part] is not actually but potentially the forms [of things].