What is a material thing made of?
Thomas Aquinas and Aristotle were so convinced of the intrinsic unity of substances, that they believed that the matter of which substances are made cannot actually contain any other substances. It is apparent that material things have parts which are recognizable as being the sorts substances which also stand alone and exist when not parts of any substances. However, both Aristotle and Aquinas believe for such parts to have the same substantial unity when they are parts of other things as they do when they are not parts would entail that the things these parts come together to compose are not substances.
For instance, one can see that an animal has parts that appear to be composed of material things that definitely are substances when they are not being parts of animals; e.g. bone looks like white stone, and both Aristotle and Aquinas thought it was composed of earth, which for them is one of the four basic elements. Today, we would say that material composition of bone consists of calcium, or mostly of calcium. But to say that the bones of an animal are actually earth or calcium would be to say that the animal is not itself a substance. Their point is that if one admits that the material constituents of bone are actually substances, which are characterized by an intrinsic unity, then whatever they are the constituents of is not a itself substantial unity, but an accidental unity, analogous to a heap of stones, or a machine that is pieced together.
They believed this because it appeared to them contradictory to say that the parts of substances are actually substances. Anything you call a substance cannot actually have other substances as parts for then two substances will be in the same place at the same time, and the same thing will be two things, in the same respect, i.e. as an actual substance. Thus, a certain piece of matter, say a bone, will be actually and substantially both an animal and earth (a non-animal) at the same time; but this is logically not possible.
Instead of being parts of other substances with the full actuality of substances, Aristotle and Aquinas said that these other substance, i.e. the elements, become the proximate matter to the substances into which they changed.
Since, however, some things are-potentially while others are-actually, the constituents combined in a compound can ‘be’ in a sense and yet ‘not-be’. The compound may be-actually other than the constituents from which it has resulted; nevertheless, each of them may be-potentially what it was before they were combined, and both of them may survive undestroyed. …The constituents, therefore, neither persist actually, as ‘body’ and ‘white’ persist: nor are they destroyed (either one of them or both), for their ‘power of action’ is preserved.Aristotle, On Generation and Corruption, I. 10 (327b23-27,30-32
Since ‘matter’ is the principle of potency, the matter of the elements becomes the matter of the substance they compose, but the elements are present potentially in the newly composed substance. So, while there is only one substance that results from the composition of various elements, the new substance has the powers of the elements which came together in its composition. Aquinas thus says that the elements are not actually in the substance, but they are there virtually, i.e. by their power (virtus).
Therefore we must say, in accordance with the Philosopher, that forms of the elements remain in the mixed body, not actually, but virtually. For the proper qualities of the elements remain, though modified; and in these qualities is the power of the elementary forms. This quality of the mixture is the proper disposition for the substantial form of the mixed body; for instance, the form of a stone, or of any sort of soul.ST Ia, 76, 4 ad 4
Applying this understanding of material things to chemical theory, Thomists analyze chemical processes as, in a certain sense, irreducible, to the material constituents. Rather, even in the terms of molecules and atoms, chemical processes tend to confirm, rather than confound, the Aristotelian/Thomistic understanding.
Water is “Made of” Hydrogen and Oxygen
For example, in the chemical reaction 2H2 + O2 –> 2 H2O, a substantial change takes place. Two substances, hydrogen and oxygen, changed into a new substance, water. The change cannot be attributed merely to the rearrangement of the chemical elements involved. The two elements are present in themselves as gases before change takes place. Since change does occur, some different substance comes to be. If the elements caused the change, two gasses in themselves would be substantially the same as water, which they are not. Also, hydrogen and oxygen are rearranged to make things other than water, eg. hydrogen peroxide, H2O2. These elements in themselves are not the reason why hydrogen and oxygen when combined produce water. The change, then, is explained in terms of prime matter and substantial form.
The two gasses, hydrogen and oxygen, are material substances. They change into another material substance, water. All of the substances involved are material; that is, they all have prime matter. Each substance is different; that is, they each have a different substantial form. The essence of water is different from the essence of either hydrogen or oxygen, and each of these essences is different from the other. The change of gasses into water is explained by these principles of prime matter and substantial form. The prime matter of the gasses persists throughout the change. It is in itself the potentiality to have any substantial form, although it never exists in reality without one. So the same prime matter that had the form of oxygen in one body and the form of hydrogen in another body gives up these forms in the change and takes the form of water. What is different from the beginning at the end of the change is the form that the same prime matter has.
In this change, water does not arise from the lack of water, but from the lack of the form water in other material substances. Prime matter which allows things to become other things is not merely the lack of the new thing. It is the lack of the form of the new thing in the old thing. Or more precisely, it is the potentiality for forms in bodies.
The elements themselves have both prime matter and substantial form. The prime matter is the principle that makes these things to be material and allows them to change, but it is not the elements themselves. The material substances are not the result of the combination of elements, but the combination of elements in material substances is the result of the substantial form that prime matter has. Water may be described as H2O because it is still has the powers and qualities of the elements, e.g. mass; the elements of hydrogen and oxygen are present virtually in water. However, water is a single substance, and the substance of water has the form of water in prime matter, the elements of hydrogen and oxygen being the proximate matter to water.