Objection 1. It seems that the definition of eternity given by Boethius (De Consol. v) is not a good one: "Eternity is the simultaneously-whole and perfect possession of interminable life." For the word "interminable" is a negative one. But negation only belongs to what is defective, and this does not belong to eternity. Therefore in the definition of eternity the word "interminable" ought not to be found.
Objection 2. Further, eternity signifies a certain kind of duration. But duration regards existence rather than life. Therefore the word "life" ought not to come into the definition of eternity; but rather the word "existence."
Objection 3. Further, a whole is what has parts. But this is alien to eternity which is simple. Therefore it is improperly said to be "whole."
Objection 4. Many days cannot occur together, nor can many times exist all at once. But in eternity, days and times are in the plural, for it is said, "His going forth is from the beginning, from the days of eternity" (Micah 5:2); and also it is said, "According to the revelation of the mystery hidden from eternity" (Romans 16:25). Therefore eternity is not omni-simultaneous.
Objection 5. Further, the whole and the perfect are the same thing. Supposing, therefore, that it is "whole," it is superfluously described as "perfect."
Objection 6. Further, duration does not imply "possession." But eternity is a kind of duration. Therefore eternity is not possession.
I answer that, As we attain to the knowledge of simple things by way of compound things, so must we reach to the knowledge of eternity by means of time, which is nothing but the numbering of movement by "before" and "after." For since succession occurs in every movement, and one part comes after another, the fact that we reckon before and after in movement, makes us apprehend time, which is nothing else but the measure of before and after in movement. Now in a thing bereft of movement, which is always the same, there is no before or after. As therefore the idea of time consists in the numbering of before and after in movement; so likewise in the apprehension of the uniformity of what is outside of movement, consists the idea of eternity.
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 to Objection 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 to Objection 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 to Objection 3. Eternity is called whole, not because it has parts, but because it is wanting in nothing.
Reply to Objection 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 to Objection 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 to Objection 6. Whatever is possessed, is held firmly and quietly; therefore to designate the immutability and permanence of eternity, we use the word "possession."
Source: Summa Theologica (New Advent)