Chapter 79 — That the substance through which man understands is the lowest in the genus of intellectual substances

Infinite progression is impossible in any series. Among intellectual substances, one must ultimately be found to be supreme: namely, the one which approaches most closely to God. Likewise, one must be found to be the lowest, and this will be the one most intimately associated with bodily matter. This can be explained in the following way. Understanding is proper to man beyond all the other animals. Evidently, man alone comprehends universals, and the relations between things, and immaterial things, which are perceptible only to the intelligence. Understanding cannot be an act performed by a bodily organ, in the way that vision is exercised by the eye. For no faculty endowed with cognitive power can belong to the genus of things known through its agency: thus the pupil of the eye lacks color by its very nature. Colors are recognized to the extent that the species of colors are received into the pupil; but a recipient must lack what it receives. Now, the intellect is capable of knowing all sensible natures. Therefore, if it knew through the medium of a bodily organ, that organ would have to be entirely lacking in sensible nature; but this is impossible.

Moreover, every cognitive faculty knows according to the way the species of the object known is in it, for this is its principle of knowing. But the intellect knows things immaterially, even those things that are by nature material, by abstracting a universal form from its individuating material conditions. Therefore, the species of the object known cannot exist in the intellect materially; and so it is not received into a bodily organ, seeing that every bodily organ is material.

The same is clear from the fact that a sense is weakened and injured by excessively strong sensible objects. Thus the ear is impaired by excessively loud sounds, and the eye by excessively bright lights, because the harmony within the organ is shattered. The intellect, on the contrary, is perfected by the excellence of intelligible objects; he who understands the higher objects of intelligence is able to understand other objects more, rather than less, perfectly. Consequently, if man is found to be intelligent, and if man’s understanding is not through a bodily organ, there must be some non-bodily substance whereby man exercises the act of understanding. For the substance of a being that can perform an action by itself, without the aid of a body, is not dependent on a body. But all powers and forms that are unable to subsist by themselves without a body cannot exercise any activity without a body. Thus heat does not cause warmth by itself; rather, a body causes warmth by the heat that is in it. Accordingly, this non-bodily substance whereby man understands occupies the lowest place in the genus of intellectual substances, and is the closest to matter.