Clearly, therefore, no succession occurs in God. His entire existence is simultaneous. Succession is not found except in things that are in some way subject to motion; for prior and posterior in motion cause the succession of time. God, however, is in no sense subject to motion, as has been shown. Accordingly there is no succession in God. His existence is simultaneously whole.
Again, if a being’s existence is not simultaneously whole, something can be lost to it and something can accrue to it. That which passes is lost, and what is expected in the future can be acquired. But nothing is lost to God or accrues to Him, since He is immutable. Therefore His existence is simultaneously whole.
From these two observations the proper meaning of eternity emerges. That is properly eternal which always exists, in such a way that its existence is simultaneously whole. This agrees with the definition proposed by Boethius: “Eternity is the simultaneously whole and perfect possession of endless life.”
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Absence of Succession in God, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
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