Aeviternity (or “the aevum”) is like time:
1.) Aeviternal things are created and so transition from not existing to existing. Setting aside whether it is a real possibility, annihilation is a logical possibility in the aevum.
2.) Something in the aevum corresponds to the future contingents in time. If not, all time would exist in a single present moment in the aevum, making it eternity. For the same reason, something in the aevum corresponds to things past in time.
Aeviternity is unlike time:
1.) Time measures what has an intrinsic ability to be another substance, aevum measures what has no such intrinsic ability. Though creation and annihilation are possible in the aevum, things are not made from what had an ability to become them, nor could an aeviternal substance be repurposed to make another substance or organism like we make trees into lumber or cows into protein sources.
2.) Time and the aevum have no common measure. All things in time are measured by a single uniform motion, once thought to be sidereal motion (and which we still use for practical purposes of measuring time) but now thought to be whatever moves at the speed of light, which is a better and more uniform measure allowing for universal translation of all motions. Things in the aevum are not measured by sidereal motion or light, while things in the aevum do X now and Y later, asking how many seconds pass between these is like trying to figure out what age Hamlet is when he dies by measuring the run time of the play.
All measure is to something uniform, so things in the aevum are measured by whatever is most uniform among them. For immaterial things, this is the the nature understanding by the most unified concept, giving rise to a will most firmly rooted in the ultimate good.
3.) Though there is something permanent in time, it is not aeviternal. Among things in time, the measure of time is most like the aeviternal because the measure is the most permanent and unchanging. This is why the older cosmology saw the stars as permanent and unchageable, and why it is circular to ask how fast a thing is moving when moving at the speed of light.* We haven’t updated eschatology to cosmology yet, but just as Thomas once taught that the blessed souls would exist as heavenly bodies, we should now teach the blessed exist like whatever-it-is-that-moves-at-‘c’ (WITMAC?). The blessed are bodies of light while the damned are bound to matter, and matter at its most inconstant, unstable, and distant from its proper place** (i.e. fire)
In another sense, any conserved quantity is a measure of motion and this status as measure is precisely why it must be conserved, though we set aside those measures which measure only by potency.
4.) Though the measure of things in time is permanent and therefore most like the aeviternal, all that is aeviternal lacks matter as an intrinsic principle and therefore is intellectual. Even when permanent, material receives form as subjective, physical, and composite while intellect receives form as objective, supra-physical and simple.
In ancient-medieval cosmology the pure spirits are united to the universe at the point where the universe is measured. This once made them as distant as the stars but it now crowds them in as close as WITMAC. Immanent angelic activity is a sort of underwriting of the nature that measures all things.
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*C : 300,000km/sec :: meter : 100 centimeters, i.e. Both tell us something true, but in both the second term is only intelligible if the first is taken as given. Of itself, the measure is not measured.
**Fire was seen as a heavenly element due to its motion upwards. In the older cosmology, the damned were in a fire at the furthest possible distance from the heaven, sc. the center of the earth. The sense is clear: though made with a desire for the highest, they are bound in place to the lowest.