Eternity, Aeviternity, and Time
Glossary
Eternity, Aeviternity, and Time
By Stephen Michael Leininger
Posted: 11/23/2025
STOSSBooks
The Eternal NOW
Eternity, Aeviternity, & Time
Without understanding eternity to some degree, we cannot grasp the hypostatic union of Jesus’ humanity with the Divine Person of the Son of God. Said understanding is essential for interpreting biblical events such as the Transfiguration, the Burning Bush, the Back Side of God, God walking in the Garden, and much more. So, let us explore that understanding. Aquinas provides us with a good definition of eternity.
He writes:
As we attain to the knowledge of simple things [e.g., forms] by way of compound things [e.g., prime substances], so we must 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 and 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 (Thomas Aquinas, A Summa of the Summa: The Essential Philosophical Passages of St. Thomas Aquinas’ Summa Theologica, ed. Peter Kreeft (San Francisco: Ignatius Press, 1990), pp. 107–108).
Through Aquinas’ explanation, we can better understand the phrase, the eternal now of God.
Aquinas goes on to say:
The now that stands still, is said to make eternity according to our [very limited] apprehension. As the apprehension of time is caused in us by the fact that we apprehend the flow of the now; so the apprehension of eternity is caused in us by our apprehending the now standing still. . . . His eternity includes all times; not as if He Himself were altered through present, past, and future (Aquinas, A Summa of the Summa, pp. 107–108).
In an endnote in the source-book which cites the above quote, Peter Kreeft adds further explanation. He writes, “We think of eternity as negative, as excluding time (for “eternal” means, after all, “not-temporal”); but in objective fact, God’s eternity is positive and includes all times, for it can lack nothing actual, positive, and perfect that time contains. A remote parallel: the mind of an author contains, at once, all his characters and the events of his plot” (Aquinas, A Summa of the Summa, pp. 107–108).
All of the above quotes are very important for understanding the goal of this blog post.
Whether Eternity Differs from Time?
According to Peter Kreeft:
Eternity is simultaneously whole. But time has a before and an after. Therefore, time and eternity are not the same thing. . . . It is manifest that time and eternity are not the same. Some have founded this difference on the fact that eternity has neither beginning nor an end; whereas time has a beginning and an end. This, however, makes a merely accidental, and not an absolute difference; because, granted that time always was and always will be, according to the idea of those who think the movement of the heavens goes on forever, there would yet remain a difference between eternity and time, as Boëthius says (De Consol. v), arising from the fact that eternity is simultaneously whole (Aquinas, A Summa of the Summa, pp. 107–108).
St. Hildegard spoke of creation thus:
When God said, “Let it be done!” things were enclosed at once within their forms [including the glorified body of the Son of God] [Note: at the instant of creation, i.e., the Big Bang, time commenced]. . . . Just as everything in front of a mirror shines within that mirror, all the works of the holy Godhead shine within it in a timeless way. For how should God exist without having prior knowledge of the divine works [creation]? And each divine work, once it has been enclosed within its body, is complete in the function that is appropriate for it. For the holy Godhead knew in advance how it would assist that work, serving it with knowledge and comprehension (Hildegard of Bingen, Hildegard of Bingen’s Book of Divine Works: With Letters and Songs, ed. Matthew Fox, trans. Robert Cunningham (Santa Fe, NM: Inner Traditions/Bear & Company), Kindle Edition, Locations 575-590; ebook ISBN: 978-1-59143-818-2).
The Difference between Aeviternity and Time
Aquinas writes:
Aeviternity differs from time, and from eternity, as the mean between them both. This difference is explained by some to consist in the fact that eternity has neither beginning nor end, aeviternity, a beginning but no end, and time both beginning and end. This difference, however, is but an accidental one. . . . This [aeviternity] appears in the heavenly bodies, the substantial being of which is unchangeable; and yet with unchangeable being they have changeableness of place. The same applies to the angels, who have an unchangeable being as regards their nature with changeableness as regards choice; moreover they have changeableness of intelligence, of affections, and of places, in their own degree. Therefore these are measured by aeviternity, which is a mean between eternity and time. But the being that is measured by eternity is not changeable, nor is it annexed to change. In this way time has before and after; aeviternity in itself has no before and after, which can, however, be annexed to it; while eternity has neither before nor after, nor is it compatible with such at all. . . . . Reply Obj. 2. Aeviternity is simultaneously whole; yet it is not eternity, because before and after are compatible with it (Aquinas, A Summa of the Summa, pp. 108-110).