Why God Cannot Be a Primary Substance

That God is “not just another primary substance” within the universe is an important theistic commitment, since to suggest otherwise entails God not being relatively different from the rest of creation. Denying that God is a primary substance secures a principled monotheism.

Now, to appreciate why God cannot be just another primary substance, we must understand what motivates the philosophical affirmation of God’s existence in the first place — which, for Thomas Aquinas, is causal reasoning. As well, we must understand what is entailed when positing a primary substance. Then, when connecting the two, we’ll see that what we affirm as God (the primary first cause) cannot be another primary substance – rather, God must be a subsistent. Really, subsistent existence itself. Let us step through this process carefully, taking the considerations in reverse order.

First, every primary substance is a composite entity. It is composite insofar as it is comprised of substance and accident. Substance refers to what stands under (the principle of unity of any changing thing) any current configuration of being that something happens to be manifesting at some time, though it is possible for it to enter into some new configuration. For example, Fido may currently be configured in a sitting position – thus, Fido has acquired a sitting accident – which actualizes a passive potential of Fido’s being, a potential which itself is within the range of possible configurations the substance of Fido can manifest. Again, Substance refers to that which stands under any current configuration of being, as the range, if you like, of possible modes of existential expression. Substance accounts for how things endure through change (size, color, place, location, etc.).

Conversely, we call an accident whatever mode of configuration is present. Some accidents are contingent (like my having brown hair), whereas other accidents are more closely tied to the substance (like my being able to laugh) having a status closer to – yet not identical with – necessity. The latter accidents are called proper accidents or “propria.” Propria are expected to manifest “always or for the most part” given respective substances (really, it is through propria that we infer the substance or essence of something), the absence of which typically marks a defect. For example, whereas my not having brown hair is not marking a defect, my inability to laugh or use language would.  

Going further, we can see substance and accident relate as potency to act, which reveals another compositional structure within primary substances. Accidents actualize some passive potency in substances in the order of form, while substance actualizes the passive potencies in accidents existentially. What emerges from this analysis is a metaphysical composite with two irreducible co-principles, each of which is possibly only in reference (co-existence) with the other. Hence, any primary substance begins to seek an extrinsic cause or external unifier. How did these principles come to be united? Not just substance and accident, but act and potency, and essence and existence.

Consider the primary substance Fido. Going, now, into the engine room of Fido’s being, we see this little doggy is comprised of his essence (= what determination) and his existence (= whether determination). These co-principles relate as potency to act, as well – Fido’s essence being merely possibly until actuality is imparted from Fido’s act of existence. Here we encounter that familiar construction problem. Fido’s act of existence requires Fido’s essence to be individuated – it is, after all, the existence of Fido, not some other thing. Yet, Fido cannot do anything, let alone individuate some instance of existence, unless Fido exists. Each principle is dependent upon the other, can only possibly be if the other is already there, doing its function. By this look, it seems primary substances would be impossible. Unless, of course, there is more to the story…

St. Thomas thinks there is more to the story — of course he does. Namely, there is a cause of such composite entities, otherwise their existence would be unintelligible – impossible, at that. Composite entitles demand extrinsic unification. Ultimately, any act-potency composite must be explained by an efficient cause. If substances fall under this category, then all primary substances must find an efficient cause, as well.

Now just connect the dots with cosmological arguments for God’s existence, which typically focus on some general aspect of being – like compositeness – that would otherwise lack a sufficient condition for its existence apart from some cause or set of conditions that transcend the category at hand. Composite things, then, must ultimately be explained by something simple and non-composite: that is, by something not comprised of act and potency, but rather (since act is ontologically prior to potency) by that which is purely actual. Relatedly, for those in which there is a real distinction between essence and existence (another instance of metaphysical composition) they must ultimately be explained by that in which its essence just is its existence (another form of ontological simplicity).

We can now see why God cannot be just another primary substance within the universe. All primary substances require a cause disjoint from themselves. The fact that there is any thing of the type substance to being with demands a cause that is disjoint from that type to avoid circularity or unintelligibility. Thus, God, as primary first cause, must transcend the category of substance, and being identical with his existence, is properly called a subsistent– in fact, the sole instance of subsistent existence itself – not a substance.