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In the respondeo, St. Thomas clearly highlights the two chief aspects of the concept of eternity.
First, eternity is interminable, i.e., it has neither beginning nor end in time. Yet, this alone does not distinguish eternity from time. For, as St. Thomas teaches, we only know by faith that the world is not eternal. The classic example is that of the eternal foot in sand. The footprint would have been caused by the foot in the sand, yet, it would not have had a beginning in time. In a similar way, God could have caused creation without there being a point in time where it began.