Summa contra Gentiles3.42 Chapter XLII. That in this life we are unable to know separate substances in the manner proposed by alex- ander?

Alexander supposed the possible intellect to be subject to generation and corruption, on account of its being a disposition of human nature resulting from the mixture of the elements, as we have seen in the Second Book. Now it is not possible for a power of this kind to arise above the material world. And so he maintained that our possible intellect can never attain to the understanding of separate substances: but he held that, according to our present state of life, we are able to understand separate substances. He endeavoured to prove this as follows. Whenever a thing is completed as regards being generated, and has reached the ultimate perfection of its substance, its proper operation, whether action or passion, will also be completed: for even as operation follows substance, so does perfection of operation; follow perfection of substance: wherefore an animal, when quite perfect, is able to walk by itself. Now the habitual intellect, which is nothing else but the intelligible species formed by the active intellect, and residing in the possible in- tellect, has a twofold operation, One is to make things potentially understood to be actually understood,—and this it has in reference to the active intellect,—while the other is to understand what is actually understood: for man is ' able to do these two things by an intellectual habit. Accord- ingly when the generation of the habitual intellect is com- plete, both of these operations will be completed in the intellect. Now whenever the intellect acquires new species it reaches the complement of its generation. And so its generation must necessarily be completed eventually, unless there be an impediment: since no generation tends to the \infinite. Therefore eventually both operations will be completed in the habitual intellect, by its making all things peneneially: understood to be understood actually,—which is the complement of the first operation;—and by under- standing all things intelligible, both separate and not separate. And seeing that the possible intellect is unable to under- stand separate substances, according to his opinion, as we have stated; he means that we shall understand separate substances by the habitual intellect, in so far as the active intellect, which according to him is a separate substance, will become the form of the habitual intellect and be united. 1 Ch, xii.

to us: so that thereby we shall understand, even as now we understand by the possible intellect; and, since it is in the power of the active intellect to make things actually understood, which are intelligible potentially, and to understand separate substances, in that state we shall understand separate substances, and all non-separate intelligible things.

According to this explanation, by this knowledge which we derive from phantasms, we attain to the knowledge of the separate substance; not as though the phantasms themselves and the things understood from them, were a means for knowing separate substances, as happens in speculative sciences, which was the position of the previous opinion; but because the intelligible species are in us a kind of disposition to this particular form which is the active intellect: This is the first point of difference between these two opinions.

Consequently, when the habitual intellect becomes perfect through these intelligible species produced in us by the active intellect, the active intellect itself becomes a form united to us, as stated. And he calls this the acquired intellect, which, they state, Aristotle held to come from without. And so, although man’s ultimate perfection is not seated in the speculative sciences, as the previous opinion maintained; yet by them man is disposed for the attainment of his ultimate perfection. This is the second point of difference between the second and first opinions.

Thirdly they differ in this, that, according to the fist opinion, the active intellect’s act of understanding is the cause of its union with us. Whereas, in the second opinion, the reverse is the case: since it is because it is united to us as a form, that we understand it and other separate substances.

But there is no reason in these statements. For the habitual intellect, like the possible intellect, is supposed by Alexander to be subject to generation and corruption. Now according to him an eternal thing cannot become the form of that which can be generated and corrupted; for this is why he maintains that the possible intellect, which is united to us as a form, is subject to generation and corruption; and that the active intellect, which is incorruptible, is a separate substance. Since then, according to Alexander, the active intellect is supposed to be an eternal separate Again. The form of the intellect, as intellect, is the intelligible, just as the form of the sense is the sensible: for If then the active intellect cannot become an intelligible through the habitual intellect, it cannot possibly be its form.

Besides. There are three ways in which we are said to understand by means of something. First, we understand by means of the intellect, which is the power that elicits this operation: wherefore also the intellect may be said to understand, and the very act of the intellect in understanding becomes our act of understanding.—Secondly, we understand understand, not as though the species itself understood, the power of sight is by the species of colour.—Thirdly, as If, then, man at length understands separate substances through his intellect, it must be in one of these ways. It is active intellect——Nor is it in the second way: because to understand by means of an intelligible species is ascribed to Alexander does not grant that either the possible or the habitual intellect understands separate substances: and consequently we cannot possibly understand separate substances by means of the active intellect in the same way as we understand things by means of an intelligible species.—And if it is as by an intellective power, it follows that the active intellect’s act of understanding is the man’s act of understanding. Now this cannot be unless the substance of the active intellect and the substance of the man be joined together in unity of being: for it is impossible that there be identity of operation where there is distinction of substances. Hence the active intellect will be one in being with man. But not in respect of accidental being: because the active intellect would then be not a substance but an accident: for instance colour added to a body makes one according to accidental being. It would follow then, that the active intellect together with man makes one in substantial being. Therefore it will be either the human soul or a part thereof, and not a separate substance, as Alexander maintained. Therefore the opinion of Alexander does not explain how man can understand separate substances.

Moreover. If the active intellect at any time become the form of this particular man, so that he be able to understand by means of it, for the same reason it may become the form of some other man who will likewise understand by its means: the result being that at the same time two men will understand by means of the active intellect, as their form. But this implies that the active intellect’s act of understanding is the act of understanding of the man who understands by its means, as already stated: and consequently two who understand will have one act of understanding. Which is impossible.

Moreover his reasoning is altogether frivolous. First, because, when the generation of a genus is perfected, its operation must be perfected, yet in keeping with the mode of that, but not of a higher genus: for when the generation of air is perfected, it has generation and complete upward movement, yet not so as to be moved towards the place of fire. Likewise, when the generation of the habitual intellect is complete, its operation, which is to understand, will be complete, according to its mode, but not according to the mode of understanding in separate substances, so as to understand separate substances. Consequently from the generation of the habitual intellect it cannot be concluded that at some time man will understand separate substances.

Secondly, because it belongs to the same power to comintellect sometimes understands separate substances. But Alexander does not hold this: for it would follow that to understand separate substances comes to us through the intellect. Thirdly, those things that begin to be generated, for the most part are completely generated: since all generations of things are due to definite causes, which produce their effects either always or in the majority of cases. If then, completeness of action follows completeness of generation, it follows that complete operation accrues to things generated, always or most frequently. And yet those who study in order that habitual intellect may be engendered in them do attained to this point of perfection. Therefore the perfection