Prima Pars, Question 88 — Summa Theologiae
Article 1. Whether the human soul in the present state of life can understand immaterial substances in themselves?
Objection 1. It would seem that the human soul in the present state of life can understand immaterial substances in themselves. For Augustine (De Trin. ix, 3) says: “As the mind itself acquires the knowledge of corporeal things by means of the corporeal senses, so it gains from itself the knowledge of incorporeal things.” But these are the immaterial substances. Therefore the human mind understands immaterial substances.
Objection 2. Further, like is known by like. But the human mind is more akin to immaterial than to material things; since its own nature is immaterial, as is clear from what we have said above (I:76:1). Since then our mind understands material things, much more is it able to understand immaterial things.
Objection 3. Further, the fact that objects which are in themselves most sensible are not most felt by us, comes from sense being corrupted by their very excellence. But the intellect is not subject to such a corrupting influence from its object, as is stated De Anima iii, 4. Therefore things which are in themselves in the highest degree of intelligibility, are likewise to us most intelligible. As material things, however, are intelligible only so far as we make them actually so by abstracting them from material conditions, it is clear that those substances are more intelligible in themselves whose nature is immaterial. Therefore they are much more known to us than are material things.
Objection 4. Further, the Commentator says (Metaph. ii) that “nature would be frustrated in its end” were we unable to understand abstract substances, “because it would have made what in itself is naturally intelligible not to be understood at all.” But in nature nothing is idle or purposeless. Therefore immaterial substances can be understood by us.
Objection 5. Further, as sense is to the sensible, so is intellect to the intelligible. But our sight can see all things corporeal, whether superior and incorruptible; or lower and corruptible. Therefore our intellect can understand all intelligible substances, even the superior and immaterial.
On the contrary, It is written (Wisdom 9:16): “The things that are in heaven, who shall search out?” But these substances are said to be in heaven, according to Matthew 18:10, “Their angels in heaven,” etc. Therefore immaterial substances cannot be known by human investigation.
I answer that, In the opinion of Plato, immaterial substances are not only understood by us, but are the objects we understand first of all. For Plato taught that immaterial subsisting forms, which he called “Ideas,” are the proper objects of our intellect, and thus first and per se understood by us; and, further, that material objects are known by the soul inasmuch as phantasy and sense are mixed up with the mind. Hence the purer the intellect is, so much the more clearly does it perceive the intelligible truth of immaterial things.
But in Aristotle’s opinion, which experience corroborates, our intellect in its present state of life has a natural relationship to the natures of material things; and therefore it can only understand by turning to the phantasms, as we have said above (I:84:7). Thus it clearly appears that immaterial substances which do not fall under sense and imagination, cannot first and per se be known by us, according to the mode of knowledge which experience proves us to have.
Nevertheless Averroes (Comment. De Anima iii) teaches that in this present life man can in the end arrive at the knowledge of separate substances by being coupled or united to some separate substance, which he calls the “active intellect,” and which, being a separate substance itself, can naturally understand separate substances. Hence, when it is perfectly united to us so that by its means we are able to understand perfectly, we also shall be able to understand separate substances, as in the present life through the medium of the passive intellect united to us, we can understand material things. Now he said that the active intellect is united to us, thus. For since we understand by means of both the active intellect and intelligible objects, as, for instance, we understand conclusions by principles understood; it is clear that the active intellect must be compared to the objects understood, either as the principal agent is to the instrument, or as form to matter. For an action is ascribed to two principles in one of these two ways; to a principal agent and to an instrument, as cutting to the workman and the saw; to a form and its subject, as heating to heat and fire. In both these ways the active intellect can be compared to the intelligible object as perfection is to the perfectible, and as act is to potentiality. Now a subject is made perfect and receives its perfection at one and the same time, as the reception of what is actually visible synchronizes with the reception of light in the eye. Therefore the passive intellect receives the intelligible object and the active intellect together; and the more numerous the intelligible objects received, so much the nearer do we come to the point of perfect union between ourselves and the active intellect; so much so that when we understand all the intelligible objects, the active intellect becomes one with us, and by its instrumentality we can understand all things material and immaterial. In this he makes the ultimate happiness of man to consist. Nor, as regards the present inquiry, does it matter whether the passive intellect in that state of happiness understands separate substances by the instrumentality of the active intellect, as he himself maintains, or whether (as 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.