"God should not be called an individual substance, since the principal of individuation is matter"

Thomas Aquinas draws from Aristotle’s metaphysical framework to clarify the unique nature of God’s existence. For Aquinas, material beings are called individuals or substances because they consist of both form and matter; each material thing is a particular, thisness (haecceity), instantiated by the limitation that matter imposes. Matter, then, acts as the principle of individuation, it distinguishes one entity from another within the same species. For example, two animals of the same kind differ because each possesses matter separate from the other.

God, on the other hand, is pure actuality, pure form without any admixture of potentiality or matter. God is ipsum esse subsistens, subsistent being itself, utterly simple and undivided. Since God has no material component, He cannot be distinguished from anything else by matter; there is, in fact, no “other” within the same genus as God. To refer to God as “an individual substance” is to misapply a category appropriate only to finite, composite beings.

Aquinas’ reasoning rests on the denial of multiplicity in God. If God were individuated by matter, He would be one among many, and would owe His distinction from others to something external, namely matter. Yet God is utterly unique, infinite, and not a member of a class or kind. Describing God as an individual substance implies that the basic principles governing created things also apply to the divine nature, which for Aquinas would fatally compromise the doctrine of divine simplicity and transcendence.

Thus, for Aquinas, God transcends the order of material substances. Individual substances are limited, defined, and distinguished by their composition, especially by the restriction imposed by matter. God, being wholly immaterial and absolutely simple, cannot be called an individual substance. Speaking of God in these terms is philosophically inaccurate, because God’s very mode of existence is infinitely different from that of all finite beings.