The doctrine that God is absolutely simple derives from the metaphysical considerations that God is a being whose existence is self-explanatory, absolutely perfect, and pure actuality. Prior to Thomas, the doctrine has its most influential formulations in Augustine and Anselm.
According to Thomas, God is his essence and his existence. If the existence of a thing differs from its essence, this existence must be caused either by some exterior agent or by its essential properties. The latter seems to be impossible; for nothing, if caused to exist, can be the sufficient and efficient cause of its own existence. Nothing can be self-caused and thus the latter option is insufficient. Therefore, if existence differs from essence then another being must cause existence. This option is also an insufficient explanation for God’s essence and existence because another being cannot cause God because he is the first efficient cause—the uncaused cause.
There are three important claims Thomas commits to concerning the doctrine of divine simplicity.
(1) It is impossible that God have any spatial or temporal parts that could be distinguished from one another as here rather than there or as now rather than then, and so God cannot be a physical entity.
(2) It is impossible that God have any accidental properties.
(3) All of God’s intrinsic properties must be essential to him, it must be acknowledge that whatever can be intrinsically attributed to God must in reality just be the unity that is his essence.read more »