Chapter 9 — That God is simple

Hence it is also clear that the first mover must be simple. For any composite being must contain two factors that are related to each other as potency to act. But in the first mover, which is altogether unmoveable, all combination of potency and act is impossible, because whatever is in potency is, by that very fact, movable. Accordingly, the first mover cannot be composite.

Moreover, something has to exist prior to any composite, since composing elements are naturally prior to a composite. Hence the first of all beings cannot be composite. We also observe that, in the order of things that exist, simpler things are before composite things. Thus elements are naturally prior to mixed bodies. Likewise, among the elements themselves, the first is fire, which is the simplest of all. Prior to all elements is the heavenly body, which has an even simpler construction since it is free from all contrariety. Hence the truth remains that the first of beings must be absolutely simple.