HKB
he says Alexander holds) the passive intellect can never understand separate substances (because according to him it is corruptible), but man understands separate substances by means of the active intellect.
This opinion, however, is untrue. First, because, supposing the active intellect to be a separate substance, we could not formally understand by its instrumentality, for the medium of an agent's formal action consists in its form and act, since every agent acts according to its actuality, as was said of the passive intellect (Q. 70, A. 1). Secondly, this opinion is untrue, because in the above explanation, the active intellect, supposing it to be a separate substance, would not be joined to us in its substance, but only in its light, as participated in things understood; and would not extend to the other acts of the active intellect so as to enable us to understand immaterial substances; just as when we see colors set off by the sun, we are not united to the substance of the sun so as to act like the sun, but its light only is united to us, that we may see the colors. Thirdly, this opinion is untrue, because granted that, as above explained, the active intellect were united to us in substance, still it is not said that it is wholly so united in regard to one intelligible object, or two; but rather in regard to all intelligible objects. But all such objects together do not equal the force of the active intellect, as it is a much greater thing to understand separate substances than to understand all material things. Hence it clearly follows that the knowledge of all material things would not make the active intellect to be so united to us as to enable us by its instrumentality to understand separate substances.
Fourthly, this opinion is untrue, because it is hardly possible for anyone in this world to understand all material things: and thus no one, or very few, could reach to perfect felicity; which is against what the Philosopher says (Ethic. i, 9), that happiness is a "kind of common good, communicable to all capable of virtue." Further, it is unreasonable that only the few of any species attain to the end of the species.
Fifthly, the Philosopher expressly says (Ethic. i, 10), that happiness is "an operation according to perfect virtue"; and after enumerating many virtues in the tenth book, he concludes (Ethic. i, 7) that ultimate happiness consisting in the knowledge of the highest things intelligible is attained through the virtue of wisdom, which in the sixth chapter he had named as the chief of speculative sciences. Hence Aristotle clearly places the ultimate felicity of man in the knowledge of separate substances, obtainable by speculative science; and not by being united to the active intellect as some imagined.
Sixthly, as was shown above (Q. 79, A. 4), the active intellect is not a separate substance; but a faculty of the soul, extending itself actively to the same objects to which the passive intellect extends receptively; because, as is stated (De Anima iii, 5), the passive intellect is "all things potentially," and the active intellect is "all things in act." Therefore both intellects, according to the present state of life, extend to material things only, which are made actually intelligible by the active intellect, and are received in the passive intellect. Hence in the present state of life we cannot understand separate immaterial substances in themselves, either by the passive or by the active intellect.
Reply Obj. 1: Augustine may be taken to mean that the knowledge of incorporeal things in the mind can be gained by the mind itself. This is so true that philosophers also say that the knowledge concerning the soul is a principle for the knowledge of separate substances. For by knowing itself, it attains to some knowledge of incorporeal substances, such as is within its compass; not that the knowledge of itself gives it a perfect and absolute knowledge of them.