For Descartes, size was more accidental than extension and I think that’s correct. When you say "Wood is the form, the substantial form, and the material or ‘stuff’ the wood is made
of is the matter" shows that Thomist are saying something MORE than what people usually mean by matter and the different forms it takes. For most people a piece of wood IS what it is, and it also has a certain shape. They don’t have the philosophical thought of a principle of potentiality united to an unseen form to make matter. This is purely Aquinas’s thought, and like the idea that existence is an actual quality, it could be beautifully said but I don’t know if it has reality to it…
Just because we don’t see something, doesn’t mean it is not real or doesn’t exist or doesn’t have any reality to it. A case in point, we don’t see God but we believe he exists. We don’t see the angels but we believe they exist. We don’t see even our own souls but we believe that our spiritual soul which possesses the spiritual powers of intellect and will is the form of our body, i.e., our spiritual soul animates our bodies and thus we are living beings. Accordingly, being that we don’t even see our own souls and our intellect and will, I don’t think it should come as to much of a surprise if we don’t see the substantial forms of things. Or, that we have knowledge of immaterial concepts such as human nature, the nature of wood, or justice.
Now, I don’t think anybody who is a sensible and reasonable person would deny that man exists. However, there is no part of man that is man or that we call man. For example, the hand is not man but a part of man. The point is universal concepts such as man are like this, they are immaterial and not sense observable. If no part of man is man or called man and yet we agree that man exists, then where does the idea of man come from? We may also consider that before we can identify the parts of a man, we must first have a man to identify the parts from.
It seems it was Socrates who was the first or one of the first who noticed the immateriality of universal concepts. Plato expounded on it and thus we have his theory of ideas or forms. For Plato, the immaterial ideas are more real than the things of the material world that participate in them. In a sense he is right, for God who is an immaterial being is reality itself. In the world, we find that things are constantly changing but the universal ideas do not change. Plato thought that if we are to have any scientific knowledge at all, our universal ideas and concepts must have objective reality to them. For if everything was just in a state of change but nothing permanent in them, we could not have any stable or scientific knowledge. Now Plato held that the ideas or forms have a separate immaterial existence apart from the material things of this world that participate in the ideas. Aristotle, on the other hand, placed the ideas or forms of Plato as a substantial element of material things. Thus, we have Aristotle’s doctrine of hylemorphism, i.e., form and matter as the substantial principles of material substances. We should also take note that Plato taught the immateriality of the intellect or reason due to our knowledge of immaterial ideas and concepts.
To get back to the piece of wood. Yes, for most people a piece of wood is what it is and the idea or concept of wood answers to the formal cause. For the formal cause is that which makes a thing what it is and from which is derived the specific nature of some thing. Much like if I were to carve a statue out of a piece of wood, the piece of wood is no longer just a piece of wood but a statue of wood. The statue is an accidental form of the wood though, figure and shape are accidents of a material substance.
Most people probably do not have the philosophical thought that the substance of a piece of wood is composed of an unseen substantial form from which is derived the idea of wood and matter. However, unconsciously they are affirming in a sense the reality of the substantial form as understood by Aristotle/Aquinas. Like the idea of man, wood is a universal immaterial concept. As in the case of man, if we were to identify or name the sensible parts of wood, biology or chemistry would probably tell us that wood is composed of several elements, for example, 2 atoms of oxygen and 4 atoms of carbon. Now, wood is neither oxygen or carbon, for these are parts of wood just as hand is a part of man. The idea of wood is not found in any of the sensible parts of wood and yet we call it wood. So yes, most reasonable people call a piece of wood, wood. But, the very idea of wood is immaterial and if wood is wood then there is something about wood it appears that we do not observe with the senses. This something is according to Aristotle/Aquinas the substantial form by which wood is wood. Accordingly, if we were to analyze a piece of wood as a philosopher might who is interested in finding the ultimate principles and causes of things, the very idea of wood turns out to be a universal immaterial concept, probably unbeknownst to your average person who nevertheless understands that when they see a piece of wood, it is wood and they call it wood.
By the way, the doctrine of hylemorphism is not purely Aquinas’ thought. This doctrine is derived from Aristotle who in a way synthesized the philosophy of Plato, Socrates, and the earlier greek philosophers.
