Objection 1. It would seem that the body of the first man was not made of the slime of the earth. For it is an act of greater power to make something out of nothing than out of something; because "not being" is farther off from actual existence than "being in potentiality." But since man is the most honorable of God's lower creatures, it was fitting that in the production of man's body, the power of God should be most clearly shown. Therefore it should not have been made of the slime of the earth, but out of nothing.
Objection 2. Further, the heavenly bodies are nobler than earthly bodies. But the human body has the greatest nobility; since it is perfected by the noblest form, which is the rational soul. Therefore it should not be made of an earthly body, but of a heavenly body.
Objection 3. Further, fire and air are nobler than earth and water, as is clear from their subtlety. Therefore, since the human body is most noble, it should rather have been made of fire and air than of the slime of the earth.
Objection 4. Further, the human body is composed of the four elements. Therefore it was not made of the slime of the earth, but of the four elements.
On the contrary, It is written (Genesis 2:7): "God made man of the slime of the earth."
I answer that, As God is perfect in His works, He bestowed perfection on all of them according to their capacity: "God's works are perfect" (Deuteronomy 32:4). He Himself is simply perfect by the fact that "all things are pre-contained" in Him, not as component parts, but as "united in one simple whole," as Dionysius says (Div. Nom. v); in the same way as various effects pre-exist in their cause, according to its one virtue. This perfection is bestowed on the angels, inasmuch as all things which are produced by God in nature through various forms come under their knowledge. But on man this perfection is bestowed in an inferior way. For he does not possess a natural knowledge of all natural things, but is in a manner composed of all things, since he has in himself a rational soul of the genus of spiritual substances, and in likeness to the heavenly bodies he is removed from contraries by an equable temperament. As to the elements, he has them in their very substance, yet in such a way that the higher elements, fire and air, predominate in him by their power; for life is mostly found where there is heat, which is from fire; and where there is humor, which is of the air. But the inferior elements abound in man by their substance; otherwise the mingling of elements would not be evenly balanced, unless the inferior elements, which have the less power, predominated in quantity. Therefore the body of man is said to have been formed from the slime of the earth; because earth and water mingled are called slime, and for this reason man is called 'a little world,' because all creatures of the world are in a way to be found in him.
Reply to Objection 1. The power of the Divine Creator was manifested in man's body when its matter was produced by creation. But it was fitting that the human body should be made of the four elements, that man might have something in common with the inferior bodies, as being something between spiritual and corporeal substances.
Reply to Objection 2. Although the heavenly body is in itself nobler than the earthly body, yet for the acts of the rational soul the heavenly body is less adapted. For the rational soul receives the knowledge of truth in a certain way through the senses, the organs of which cannot be formed of a heavenly body which is impassible. Nor is it true that something of the fifth essence enters materially into the composition of the human body, as some say, who suppose that the soul is united to the body by means of light. For, first of all, what they say is false--that light is a body.
Secondly, it is impossible for something to be taken from the fifth essence, or from a heavenly body, and to be mingled with the elements, since a heavenly body is impassible; wherefore it does not enter into the composition of mixed bodies, except as in the effects of its power.
Reply to Objection 3. If fire and air, whose action is of greater power, predominated also in quantity in the human body, they would entirely draw the rest into themselves, and there would be no equality in the mingling, such as is required in the composition of man, for the sense of touch, which is the foundation of the other senses. For the organ of any particular sense must not actually have the contraries of which that sense has the perception, but only potentially; either in such a way that it is entirely void of the whole "genus" of such contraries--thus, for instance, the pupil of the eye is without color, so as to be in potentiality as regards all colors; which is not possible in the organ of touch, since it is composed of the very elements, the qualities of which are perceived by that sense--or so that the organ is a medium between two contraries, as much needs be the case with regard to touch; for the medium is in potentiality to the extremes.
Reply to Objection 4. In the slime of the earth are earth, and water binding the earth together. Of the other elements, Scripture makes no mention, because they are less in quantity in the human body, as we have said; and because also in the account of the Creation no mention is made of fire and air, which are not perceived by senses of uncultured men such as those to whom the Scripture was immediately addressed.
Source: Summa Theologica (New Advent)