4. Plato supposed knowledge in us to be caused by Ideas, which he took to subsist apart by themselves. But clearly the first principle on which our knowledge depends is the active intellect. If therefore the active intellect is something subsisting apart by itself, the difference will be none, or but slight, between this opinion and that of Plato, which the Philosopher rejects.
5. If the active intellect is an intelligence subsisting apart, its action upon us will either be continual and uninterrupted, or at least we must say that it is not continued or broken off at our pleasure. Now its action is to make the impressions on our phantasy actual terms of intelligence. Either therefore it will do this always or not always. If not always, still it will not do it at our discretion. Either therefore we must be always in the act of understanding, or it will not be in our power actually to understand when we wish.*
But it may be said that the active intellect, so far as with it lies, is always in action, but that the impressions in our phantasy are not always becoming actual terms of intelligence, but only when they are disposed thereto; and they are disposed thereto by the act of the cogitative faculty, the use of which is in our power; and therefore actually to understand is in our power; and this is why not all men understand the things whereof they have the impressions in their phantasy, because not all have at command a suitable act of the cogitative faculty, but only those who are accustomed and trained thereto.*
But this answer does not appear to be altogether sufficient. That the impressions in phantasy are marshalled by the cogitative faculty to the end that they may become actual terms of understanding and move the potential intellect, does not seem a sufficient account, if it be coupled with the supposition of the potential intellect being a separately subsistent intelligence. This seems to go with the theory of those who say that inferior agents supply only predispositions to final perfection, but that final perfection is the work of an extrinsic agency: which is contrary to the mind of Aristotle:* for the human soul does not appear to be worse off for understanding than inferior natures are for their own severally proper activities.
9. In the nature of every cause there is contained a principle sufficient for the natural operation of that cause. If the operation consists in action, there is at hand an active principle, as we see in the powers of the vegetative soul in plants. If the operation consists in receiving impressions, there is at hand a passive principle, as we see in the sentient powers of animals. But man is the most perfect of all inferior causes; and his proper and natural operation is to understand, an operation which is not accomplished without a certain receiving of impressions, inasmuch as every understanding is determined by its object; nor again without action, inasmuch as the intellect makes potential into actual terms of understanding. There must therefore be in the nature of man a proper principle of both operations, to wit, both an active and a potential intellect, and neither of them must be separate in being (or physically distinct), from the soul of man.
10. If the active intellect is an intelligence subsisting apart, it is clearly above the nature of man. But any activity which a man exercises by mere virtue of a supernatural cause is a supernatural activity, as the working of miracles, prophecy, and the like effects, which are wrought by men in virtue of a divine endowment. Since then man cannot understand except by means of the active intellect, it follows, supposing that intellect a separately subsistent being, that to understand is not an operation proper and natural to man; and thus man cannot be defined as intellectual or rational.
11. No agent works except by some power which is formally in the agent as a constituent of its being. But the working both of potential and of active intellect is proper to man: for man produces ideas by abstraction from phantasms, and receives in his mind those ideas; operations which it would never occur to us to think of, did we not experience them in ourselves. The principles therefore to which these operations are attributable, namely, the potential and the active intellect, must be faculties formally existing in us.
12. A being that cannot proceed to its own proper business without being moved thereto by an external principle, is rather driven to act than acts of itself. This is the case with irrational creatures. Sense, moved by an exterior sensible object, makes an impression on the phantasy; and so in order the impression proceeds through all the faculties till it reaches those which move the rest. Now the proper business of man is to understand; and the prime mover in understanding is the active intellect, which makes intellectual impressions whereby the potential intellect is impressed; which potential intellect, when actualised, moves the will. If then the active intellect has a separate subsistence outside man, the whole of man's activity depends on an extrinsic principle. Man then will not be his own leader, but will be led by another; and thus will not be master of his own acts, nor deserve praise nor blame; and the whole of moral science and political society will perish: an awkward conclusion. Therefore the active intellect has no subsistence apart from man.
2.75 : Confutation of the Arguments which seem to prove the Unity of the Potential Intellect
2.77 : That it is not Impossible for the Potential and the Active Intellect to be united in the one Substance of the Soul