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After affirming (abstracted from creation) that God has the entitative disposition of not having spacial limitation, St. Thomas distinguishes between two ways of "being in a place" in order to treat his actual presence in creation by way of ubiquity.
In the first way, something is present relatively and by way of other things, as the intellect is present to the stars when it gazes upon them (this is why the intellect is in a way "all things" according to the philosopher). In this way, we affirm that God is present in that He conserves all things in being.
In the second way, something is present in the way proper to a place, i.e., absolutely, as I am present to the chair I am currently sitting in, i.e., filling it. In this way too is God present to all things.
Sed Contra: It is written, I fill heaven and earth (Jer 23:24).
Respondeo: Since place is a thing, to be in place can be understood in a twofold sense; either by way of other things—i.e., as one thing is said to be in another no matter how; and thus the accidents of a place are in place; or by a way proper to place; and thus things placed are in a place.
Now in both these senses, in some way God is in every place; and this is to be everywhere. First, as He is in all things giving them being, power and operation; so He is in every place as giving it existence and locative power.
Again, things placed are in place, inasmuch as they fill place; and God fills every place; not, indeed, like a body, for a body is said to fill place inasmuch as it excludes the co-presence of another body; whereas by God being in a place, others are not thereby excluded from it; indeed, by the very fact that He gives being to the things that fill every place, He Himself fills every place.