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We begin our ascent to the metaphysical essence of God (from which we may derive all the
attributes of God) with a a division between God as "body" vs "spirit." For, a body is defined,
broadly, as a substance that is extended in three dimensions.
This is de fide from Vatican I.
In the respondeo, he gives three proofs, considering "body" under three aspects.
First, from body as physical entity. For, all bodies do not move unless they are put in motion. Yet, we have already proven that God is the first mover. Therefore...
Second, from body under the aspect of quantity. For, all things with quantity are in potentiality
(since they are potentially divisible). Yet, God is pure act since whatever is in potentiality can only
be reduced to actuality by a being in actuality (now...God is absolute prior to all and thus cannot
be reduced). Therefore...