Summa contra Gentiles3.43 Chapter XLIII. That we cannot understand separate substances in this life, in the manner suggested by Averroes!

corruptible, Averroes thought to offer an easier proof that stantially separate from us, just as the active intellect. First, active intellect to those principles which we know naturally For the habitual intellect whereby we understand, has not only this action which is to understand, but also this which is to make things actually understood: for we know by experience that both are in our power. Now “to make things actually understood,” indicates more specially the habitual intellect than “to understand’’: because it is necessary to make a thing actually understood before one understands it. Now, in us certain things are made actually understood naturally, and not by study or by choice, as the first intelligible principles. And it does not belong to the habitual intellect to make these actually understood, for it belongs to this power to make actually understood those things which we know by study: rather are they a beginning of the habitual intellect, wherefore Aristotle gives the name of understanding to the habit of these principles (6 Ethic. vi.). And they are made actually understood by the active intellect alone: and by them those other things are made actually understood which we know by study. Accordingly to make these things which are by way of consequence understood actually, is an act both of the habitual intellect, as to first principles, and of the active intellect.

Now one action does not proceed from two principles unless one of them be compared to the other as agent to instrument, or as form to matter. Consequently the active intellect must be compared to the first principles of the habitual intellect, either as agent to instrument, or as form to matter.

How this may be possible he explains as follows. Since the possible intellect, according to his opinion, is a separate substance,! it understands the active intellect and other separate substances, as well as the first principles of speculative knowledge: and consequently it is the subject of both. Now whenever two things come together in one subject, one of them is as the form of the other: even so, since colour and light are in the diaphanous body as their subject, one of them, namely light, must be the form of the other, namely colour. And this is necessary when one of them is ordained to the other, but not when they are united accidentally in the same subject, as whiteness and music. Now the object of speculative knowledge and the active intellect are mutually ordained to each other: since these understood speculative principles are made actually understood by the active intellect. Therefore the active intellect is related to these understood speculative principles as form to matter. Consequently since these same principles are joined to us by phantasms, which are a kind of subject thereof, it follows that the active intellect also is joined to us, being the form of these principles. When therefore these principles are in us potentially only, the active intellect is only joined to us potentially. When some of these principles are in us actually and some potentially, the active intellect is joined to us actually in part, and potentially in part: and then it is said to be moved towards the above conjunction: because as the more things are made actually understood in us, the more perfectly is the active intellect joined to us. /And this progress and movement towards conjunction is ¢ffected by study in speculative sciences, whereby we acquire true knowledge, and false opinions are put aside, which are outside the order of this movement, just as monstrosities are outside the operation of nature. Wherefore men help one another towards this progress, just as they help one another in speculative sciences.

us, the active intellect will be perfectly joined to us as a form, and we shall understand perfectly by it, just as now we understand perfectly by the habitual intellect. Consequently, since it belongs to the active intellect to understand separate substances, we shall then understand separate substances, just as now we understand speculative knowledge. This will be man’s ultimate happiness, wherein man will be a god as it were.

That this explanation is of no account whatever is made clear enough by what we have already said: for it is based on many suppositions that have been already disproved.

First, we have shown above that the possible intellect is not a substance distinct from usin being. Hence it does not follow that it is the subject of separate substances: especially since Aristotle asserts’ that the possible intellect is the power of becoming all things, so that seemingly it is the subject of such things only as are made to be understood.

_ Again. It has also been proved above? that the active intellect is not a separate substance, but part of the soul, to which Aristotle assigns the operation of making things to be actually understood, which lies in our power. Hence it does not follow that understanding by means of the active intellect is the cause of our being able to understand separate substances: else we would always understand them.

Further. If the active intellect were a separate substance, it would not be joined to us except by means of species made to be actually understood, according to his explanation: as neither would the possible intellect be united to us: although the possible intellect is related to those species as matter to form, while the active intellect, on the contrary, is related to them as form to matter. Now the species which are made to be actually understood are joined to us, according to him, by means of the phantasms, which stand related to the possible intellect as colours to the sight, but to the active intellect as colours to the light, as may be gathered from the statement of Aristotle (3 De Anima v.). Now we cannot ascribe to a stone in which colour is, either the action of seeing, so that it see; nor the action of the sun, so that it give light. Therefore according to this opinion, we cannot ascribe to man either the action of the possible intellect so that he understand, or the action of the active intellect, so that he understand separate substances, or that he make things to be actually understood.

Besides. According to this opinion, the active intellect is not supposed to be joined to us as a form, except through its being the form of the principles of understanding, whereof it is stated to be the form also because the active intellect and these principles have an action in common, namely to make things actually understood. Consequently it cannot be a form to us, except for as much as the principles of knowledge have an action in common with it. But these principles have no share in the action of understanding separate substances, because they are species of sensible things: unless we return to the opinion of Avempace, that the quiddities of separate substances can be known by means of what we know of the sensible world. Therefore nowise can we understand separate substances by this means.

Moreover. The active intellect bears a different relation to principles of knowledge whereof it is the cause; and to separate substances, whereof it is not the cause, but which it only knows, according to histheory. Therefore if it be joined to us through being the cause of principles of knowledge, it does not follow that it is joined to us, in so far as it knows separate substances: and clearly his argument contains a fallacy of accident.

Again. If we know separate substances by means of the active intellect, this is not because the active intellect is the form of this or that principle of understanding, but through becoming a form in us: for it is thus that we are able to understand by its means. Now it becomes a form in us also by means of the first principles of understanding, according to his own statement. Therefore from the very beginning active intellect. If, however, it be said that the active intellect does not become a form in us perfectly by means of some principles of understanding, so that we be able to understand separate substances:—the sole reason for this is because these principles of understanding do not equal the perfection of the active intellect in understanding separate substances. But not even all these principles of understanding combined together equal this perfection of the active intellect in understanding separate substances: since all of them are not intelligible except in so far as they are made to be actually understood: whereas the latter are intelligible by their very nature. Therefore although we shall know all these intelligible principles, it does not follow that the active intellect will become a form in us so perfectly that we understand separate substances by it. Else, if this be not required, we shall have to admit that by understanding anything intelligible, we also understand separate substances.