1. In the second article, Thomas asks whether God is composed of matter and form. We will pause here to lay out some basic considerations on matter and form for the reader.
2. Aristotle famously distinguishes four "causes" (Greek: aitiai) or ways of explaining things. One can think of these four causes as different ways of answering the question "why is it?" The efficient cause answers the question "What brought this thing into being?" The final cause answers the question "what perfects this thing? what is it for?" The formal cause answers the question "what is it that makes this thing itself? what makes it different in kind from other things?" The material cause answers the question "what is this thing made of? What are its parts?"
3. Of these four causes, the final and efficient causes are easiest to understand to the contemporary mind, perhaps because we are so immersed in techne that the notions of purpose and making are obvious to us. Formal and material are more difficult. The danger in a society the predominant metaphysics of which is materialist reductivism, is to assume that "material cause" means simply "atoms and subatomic particles", and that form refers either to the simple arrangement of these atomic particles, or to some mystical essence that these particles are imbued with to give them different qualities.
4. In reality, matter and form are always defined relative to each other. Matter is simply "that which receives a particular form", and form is simply "the act of a thing which makes it what it is". Imagine you are building a multi-tiered wedding cake. You begin with eggs, flour, sugar, etc. These are the matter for your batter. They become the batter by being mixed together. Here the combination and proportion of the ingredients is the form added to them to make them what they are. The batter in turn is matter for each cake layer, and it receives a certain form by being baked, to become cake layers. After the layers have baked, and the frosting and filling have been prepared, the layers, frosting, and filling become the matter for the cake as a whole, and the form is their arrangement relative to each other. When the whole cake is finished, we can ask "what is its matter?" and this question will have multiple possible answers. The proximate matter is the cake sheets, frosting and filling. More remotely, the matter is cake batter. More remotely still, the matter is eggs, flour, and sugar. But even more remotely we might say that the matter is protein and carbohydrates, atoms, protons, neutrons, electrons, quarks, mass-energy. Notice that as we choose to answer "what is it made of" with more and more remote material causes, the answer tells us less and less about the cake. If you ask what wedding cakes are made of, and I say "mass-energy", I have told you almost nothing at all about them.
5. Consider the same series of questions, except applied to the formal cause. If I bring you a wedding cake, and you ask "What is it?", I can answer this in various ways. I could say that it is a wedding cake, or that it's three sheet cakes with raspberry filling and vanilla frosting, or that it's a gallon of cake batter, some berries, butter and sugar. I could say "it's eggs, sugar, and flour", or "it's a collection of organic compounds" or "it's mass-energy". Notice, though, that saying "it's mass-energy" tells me nothing about what the cake is. The table it's resting on is also mass-energy. What distinguishes the cake from other things? What makes it different from other blobs of mass-energy, other sets of organic compounds, other sets of eggs flour and sugar, other gallons of cake batter? What makes the cake whatever it is? That's its form.
6. One further word. The Thomistic maxim is "forma dat esse". Form gives being to a thing. What does this mean? It means that every particular form which is added to a thing, every new act or new characteristic to which its matter is determined, makes that thing itself, and gives it its particular act of existence. If we remove from an object its various forms, one by one, we will eventually be left with nothing at all. This is why "prime matter", i.e. matter totally undetermined, form-less matter, does not exist on its own. Existence is, in a manner of speaking, the form of all forms. One of the greatest difficulties for the average person in understanding formal causation is the belief that being belongs exclusively to the atomic parts of things. If forms are separable from these atomic parts, the forms do not exist. On,y matter exists. What we miss in this thought is the fact that the matter receiving a particular form already has its own form, and that the form added is dependent on the form of the matter in order to be realized. Normal agents are not capable of communicating forms ex nihilo, but only giving additional act to ore existing substances. This doesn't make form purely accidental, but shows us that the communication of form is dependent on the power if the agent and the receptivity of the recipient.
7. Turning, then, to the second article of this question, we find that St. Thomas proposes three objections against the immateriality of God. First, that God is described as having a soul, but the soul is the animating principle or form of a body. (The soul or act of life is what differentiates a living body from a corpse.) Therefore God must be composed of matter and form. Second, that God is described as having emotions (wrath, etc.), and emotions are acts of the soul under the influence of the passions of the body. Third, that matter is the principle by which things with a given form are individuated from each other (this penny differs from that penny by its matter, not by its form).
8. For the Sed Contra, Thomas points out that to be composed of matter and form is to be a body, which, it has already been established, God is not.
9. In the Corpus, Thomas advances three arguments against the material composition of God. First, he points out that God is pure act, without any potency, as shown in the First Way. Since matter is by definition in potency to receive a particular form, If God were composed of matter and form he would be in potency. Second, he points out that the perfection of material things is participated, through their reception of various forms. To the extent that the matter receives the form, the composite has the perfection of the form. But as shown in the Fourth Way, God does not receive his perfection from anything, and hence cannot contain matter for the reception of some higher form. Rather, he is his own form. Third, he points out that because form is act, and matter is potency to receive act, whatever is pure and perfect act will be pure form without matter, and such is God, since he is the first agent and the ultimate efficient cause.
10. The replies to the first and second objections are, as in the previous articles, guidelines for the interpretation of scripture. God's soul is spoken of simply by way of analogy, to direct our minds to the acts of God which we associate with our souls (knowing, for example). Emotions attributed to God represent the effects of his acts: thus wrath is a symbol of the divine punishment for sin, not a description of an emotional state in God.
11. In reply to the third objection, Thomas concedes that among things that share common forms matter is the principle of individuation. Thus two identical coins are distinguished by the particular bits of metal of which they're made. Further, among forms that are shared in this way, it is matter which actualizes the form by receiving it. "A coin in general" does not exist, because coin-ness only subsists in particular individuals. But if a form is self-subsisting, then it does not need to be received in matter in order to be actualized or individuated, because it has both of these from itself. Such is God.
Outline of Article
—God's soul is referenced in scripture.
—God's emotions are described in scripture.
—Matter is the principle of individuation in things.
Sed Contra: God is not a body.
—Matter is in potency, but God has no potency.
—Matter participates in form, but God does not participate in a higher perfection.
—Form is proportionate to act, and matter to potency, but God is pure act.
–God's soul refers to his operations.
—God's emotions refer to his effects.
—Matter is the principle of individuation for material forms, but self-subsisting forms are their own principles of individuation.