Ch. 9 - "The Simplicity of God" - from "The Compendium of Theology" by Thomas Aquinas

CHAPTER 9 SIMPLICITY OF GOD

Inde etiam apparet quod oportet primum movens simplex esse. Nam in omni compositione oportet esse duo, quae ad invicem se habeant sicut potentia ad actum. In primo autem movente, si est omnino immobile, impossibile est esse potentiam cum actu; nam unumquodque ex hoc quod est in potentia, mobile est. Impossibile igitur est primum movens compositum esse.

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.

Adhuc. Omni composito necesse est esse aliquid prius: nam componentia naturaliter sunt composito priora. Illud igitur quod omnium entium est primum, impossibile est esse compositum. Videmus etiam in ordine eorum quae sunt composita, simpliciora priora esse: nam elementa sunt naturaliter priora corporibus mixtis. Item etiam inter ipsa elementa primum est ignis, quod est simplicissimum. Omnibus autem elementis prius est corpus caeleste, quod in maiori simplicitate constitutum est, cum ab omni contrarietate sit purum. Relinquitur igitur quod primum entium oportet omnino simplex esse.

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.