Summa contra Gentiles3.45 Chapter XLV. That it is impossible in this life to understand separate substances

Since then in this life separate substances cannot be known by us in the ways mentioned above,’ it remains for us to inquire whether we be able to understand separate substances in this life in any way at all.

Themistius? seeks to prove that this is possible by an argument a fortiovz. For separate substances are more intelligible than material things: since the latter are intelligible in so far as the active intellect causes them to be actually understood; whereas the former are intelligible in themselves. If, therefore, our intellect understands these material things, much more is it adapted to understand separate substances.

This argument must be appraised in the light of the various opinions about the possible intellect. For if the possible intellect is a power independent of matter, and has its being apart from the body, as Averroes maintains, it will follow that it has no necessary relation to things material; so that the more a thing is intelligible in itself, the more will it be intelligible to the possible intellect. But then it would seem to follow, since we understand from the beginning by means of the possible intellect, that we understand separate substances from the beginning: which is clearly false. Averroes seeks to avoid this difficulty, as we have explained above in setting forth his opinion, which we proved to be false.

If, however, the possible intellect is not separate from the body in its very being, from the very fact that it is united in being to such a body, it has a necessary relation to material things, so that only through them can it acquire knowledge of other things. Hence it does not follow, if separate substances be more intelligible in themselves, that they be more intelligible to our intellect. This is proved by the words of Aristotle (2 Metaph. ix.). For he says there that the difficulty of understanding those things is in us and not in them: because our intellect stands in relation to things most evident as the eye of the owl to the sunlight. Consequently, seeing that we cannot arrivé at understanding separate substances through understanding material things, as we have proved,’ it follows that our possible intellect can nowise understand separate substances.

This appears again from the relation of the possible to the active intellect. Because a passive power is in potentiality to those things only which are included in the range of its proper active principle: for every passive power has a corresponding active power in nature: otherwise passive power would be useless, since it cannot be brought to actuality, except by an active principle. Hence we find that the sight is not receptive of other than colours which are enlightened by the light. Now the possible intellect is a power in a certain sense passive, and therefore it has its corresponding agent, namely the active intellect, which stands in relation to the possible intellect as light to the sight. Consequently the possible intellect is in potentiality to those intelligible objects only which have been made so by the active intellect. Wherefore Aristotle (3 De Anima v.)

eC, xi, in describing each intellect, says that the possible intellect is the power to become all things, while the active intellect is the medium of making all things: so that the power in either case refers to the same objects, being active in the one and passive in the other. Since then separate substances are not made actually intelligible by the active intellect, and only material things are so made, it follows that the possible intellect extends to these alone: and therefore we cannot understand separate substances thereby.

Wherefore Aristotle employed a fitting example: for the owl’s eye can never see the light of the sun. And yet Averroes tries to depreciate this example, saying that the likeness between our intellect in relation to separate substances, and the owl’s eye in relation to the sun’s light, is one of difficulty, not of impossibility: and he proves this as follows. Because if it were impossible for us to understand things intelligible in themselves, separate substances to wit; they would be intelligible without purpose, as to no purpose would a thing be visible, if it could not be seen by any sight.

Now this argument is clearly of no account at all: for even though these substances be never understood by us, they are understood by themselves: so that not without purpose would they be intelligible: as neither is the sun uselessly visible——to continue Aristotle’s comparison—because the owl cannot see it, since man and other animals can see it.

Accordingly if we suppose the possible intellect to be united in being to the body, it cannot understand separate substances. It makes a difference, however, what we hold with regard to its substance. For if we suppose it to be a material force subject to generation and corruption, as some have maintained,’ it would follow that by its very substance it is confined to the understanding of material things: and consequently it would nowise be able to understand separate substances: since it could not possibly itself be separate.—On the other hand, if the possible intellect, although united to the body, be incorruptible and independent of matter in its being, as we have proved above’; it follows that its being confined to the understanding of material things is incidental to it through its union with the body. And so, when the soul shall be separated from the body, the possible intellect will be able to understand things that are intelligible in themselves, namely separate substances, by the light of the active intellect, which in the soul is like the intellectual light that is in separate substances. This is what our faith holds about our understanding separate substances after death and not in this life.