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In this first article, St. Thomas treats of God's immensity in general.
He demonstrates this by the evident fact that "that which operates 'touches' that which it operates." Since God is the cause of all being, he must touch all being.
Sed Contra: A thing is wherever it operates. But God operates in all things, according to Isa. 26:12, Lord . . . Thou hast wrought all our works in us. Therefore God is in all things.