A similar course of reasoning clearly shows 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 immobile, 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 by their very nature antecedent to a composite. Hence the first of all beings cannot be composite. Even within the order of composite beings we observe that the simpler things have priority. 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 a simpler construction, since it is free from all contrariety. Hence the truth remains that the first of beings must be absolutely simple.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Simplicity of God, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
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