Further, those things are said to be measured by time which have a beginning and an end in time, because in everything which is moved there is a beginning, and there is an end. But as whatever is wholly immutable can have no succession, so it has no beginning, and no end.
Thus eternity is known from two sources: first, because what is eternal is interminable—that is, has no beginning nor end (that is, no term either way); secondly, because eternity has no succession, being simultaneously whole.
Reply Obj. 1: Simple things are usually defined by way of negation; as "a point is that which has no parts." Yet this is not to be taken as if the negation belonged to their essence, but because our intellect which first apprehends compound things, cannot attain to the knowledge of simple things except by removing the opposite.
Reply Obj. 2: What is truly eternal, is not only being, but also living; and life extends to operation, which is not true of being. Now the protraction of duration seems to belong to operation rather than to being; hence time is the numbering of movement.
Reply Obj. 3: Eternity is called whole, not because it has parts, but because it is wanting in nothing.
Reply Obj. 4: As God, although incorporeal, is named in Scripture metaphorically by corporeal names, so eternity though simultaneously whole, is called by names implying time and succession.
Reply Obj. 5: Two things are to be considered in time: time itself, which is successive; and the "now" of time, which is imperfect. Hence the expression "simultaneously-whole" is used to remove the idea of time, and the word "perfect" is used to exclude the "now" of time.
Reply Obj. 6: Whatever is possessed, is held firmly and quietly; therefore to designate the immutability and permanence of eternity, we use the word "possession." _______________________
SECOND ARTICLE [I, Q. 10, Art. 2]
Whether God is Eternal?