Dec 26 2008
Eternity is well defined by Boethius, interminabilis vitae tota simul et perfecta possessio, the perfect and simultaneous possession of interminable life. We know what is simple by that which is composite; for we know first what is composite, through which we attain to the notion of simplicity. Accordingly, Eternity becomes known to us in a twofold manner: first, as that which belongs to Eternity is interminable, without beginning or end; and the term can be applied to both. Secondly, this Eternity is without succession, it exists all at once. Thus it exists always according to the one and same mode, and the idea of before or after has no place in it at all. God is eternal because He is Unchangeable; thus Immutability belongs to Eternity as movement belongs to time.
God is not only Eternal; He is His own Eternity; for His Duration is His Existence; as His Essence is His Existence, so is His Eternity. And God alone is Eternal, for He alone is Unchangeable. Other things share in Eternity in much the same way as they share from Him, in their own degree, of durability, as things corruptible have a long life, and thus Scripture speaks of the “eternal hills.” With some things, as the elements, this participation is held by the whole, and not according to the parts; others, like the Angels and the Blessed, participate, in a strict sense, by the substantial incorruptibility of their individual act; their happiness is in the Word, and their thoughts are not changeable. Eternity differs from Age and from Time. Eternity is without succession, which cannot be said of time; because the very notion of Time means before and after. Age differs from Time, for as Eternity is the measure of permanent existence, and is without before or after, nor can it be in a way comparable to such a notion; indeed, as a thing recedes from permanence of existence it recedes from Eternity, so also Age is without before or after in itself, but may possibly be joined to them accidentally, hence its measure is that of the heavenly bodies, the existence of which is unchangeable, although this may be joined to change as regards place. The Angels have a changeless existence, with liability to change as regards election, so far as pertains to their nature, thus being mutable as regards intelligence, affection, and place, and their measure of existence is Age. But corruptible things that recede so far from permanence of existence as to be subject to change, like all movement, are measured by Time.
Age is one; for as the oneness of Time is derived from the unity of the first movement, which is the most simple, and the rule of measurement for all others, so one Age is the measure of all others, and the more it is simply the first the more simple it is, and the principle of the rest. But many ages are reckoned as so many centuries. St Thomas Aquinas, A Compendium of the Summa