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 most intimately associated with corporeal 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 objects, 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. No faculty endowed with cognitive power can belong to the genus of things that is 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 be lacking in that which is received. 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, any cognitive faculty exercises its power of knowing in accord with the way the species of the object known is in it, for this is its principle of knowing. But the intellect knows things in an immaterial fashion, even those things that are by nature material; it abstracts 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 sensible objects of extreme intensity. Thus the ear is impaired by excessively loud sounds, and the eye by excessively bright lights. This occurs because the harmony within the organ is shattered. The intellect, on the contrary, is perfected in proportion to the excellence of intelligible objects; he who understands the higher objects of intelligence is able to understand other objects more perfectly rather than less perfectly. Consequently, if man is found to be intelligent, and if man’s understanding is not effected through the medium of a bodily organ, we are forced to acknowledge the existence of some incorporeal 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 by itself cause warmth; rather a body causes warmth by the heat that is in it. Accordingly this incorporeal substance whereby man understands, occupies the lowest place in the genus of intellectual substances, and is the closest to matter.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Inferiority of Man’s Intellectual Nature, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).