Thomas Aquinas, Summa Theologiae

Objection 1: It seems that eternity does not differ from time. For two measures of duration cannot exist together, unless one is part of the other; for instance two days or two hours cannot be together; nevertheless, we may say that a day or an hour are together, considering hour as part of a day. But eternity and time occur together, each of which imports a certain measure of duration. Since therefore eternity is not a part of time, forasmuch as eternity exceeds time, and includes it, it seems that time is a part of eternity, and is not a different thing from eternity.

Objection 2: Further, according to the Philosopher (Phys. iv), the "now" of time remains the same in the whole of time. But the nature of eternity seems to be that it is the same indivisible thing in the whole space of time. Therefore eternity is the "now" of time. But the "now" of time is not substantially different from time. Therefore eternity is not substantially different from time.

Objection 3: Further, as the measure of the first movement is the measure of every movement, as said in Phys. iv, it thus appears that the measure of the first being is that of every being. But eternity is the measure of the first being---that is, of the divine being. Therefore eternity is the measure of every being. But the being of things corruptible is measured by time. Time therefore is either eternity or is a part of eternity.

On the contrary, Eternity is simultaneously whole. But time has a "before" and an "after." Therefore time and eternity are not the same thing.

I answer that, 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 for ever, there would yet remain a difference between eternity and time, as Boethius says (De Consol. v), arising from the fact that eternity is simultaneously whole; which cannot be applied to time: for eternity is the measure of a permanent being; while time is a measure of movement. Supposing, however, that the aforesaid difference be considered on the part of the things measured, and not as regards the measures, then there is some reason for it, inasmuch as that alone is measured by time which has beginning and end in time. Hence, if the movement of the heavens lasted always, time would not be of its measure as regards the whole of its duration, since the infinite is not measurable; but it would be the measure of that part of its revolution which has beginning and end in time.

Another reason for the same can be taken from these measures in themselves, if we consider the end and the beginning as potentialities; because, granted also that time always goes on, yet it is possible to note in time both the beginning and the end, by considering its parts: thus we speak of the beginning and the end of a day or of a year; which cannot be applied to eternity. Still these differences follow upon the essential and primary differences, that eternity is simultaneously whole, but that time is not so.

Reply to Objection 1: Such a reason would be a valid one if time and eternity were the same kind of measure; but this is seen not to be the case when we consider those things of which the respective measures are time and eternity.

Reply to Objection 2: The "now" of time is the same as regards its subject in the whole course of time, but it differs in aspect; for inasmuch as time corresponds to movement, its "now" corresponds to what is movable; and the thing movable has the same one subject in all time, but differs in aspect a being here and there; and such alteration is movement. Likewise the flow of the "now" as alternating in aspect is time. But eternity remains the same according to both subject and aspect; and hence eternity is not the same as the "now" of time.

Reply to Objection 3: As eternity is the proper measure of permanent being, so time is the proper measure of movement; and hence, according as any being recedes from permanence of being, and is subject to change, it recedes from eternity, and is subject to time. Therefore the being of things corruptible, because it is changeable, is not measured by eternity, but by time; for time measures not only things actually changed, but also things changeable; hence it not only measures movement but it also measures repose, which belongs to whatever is naturally movable, but is not actually in motion.

Articulus 4

Argumentum 1

Ad quartum sic proceditur. Videtur quod aeternitas non sit aliud a tempore. Impossibile est enim duas esse mensuras durationis simul, nisi una sit pars alterius, non enim sunt simul duo dies vel duae horae; sed dies et hora sunt simul, quia hora est pars diei. Sed aeternitas et tempus sunt simul, quorum utrumque mensuram quandam durationis importat. Cum igitur aeternitas non sit pars temporis, quia aeternitas excedit tempus et includit ipsum; videtur quod tempus sit pars aeternitatis, et non aliud ab aeternitate.

Argumentum 2

Praeterea, secundum philosophum in IV physic., nunc temporis manet idem in toto tempore.

Sed hoc videtur constituere rationem aeternitatis, quod sit idem indivisibiliter se habens in toto decursu temporis. Ergo aeternitas est nunc temporis. Sed nunc temporis non est aliud secundum substantiam a tempore. Ergo aeternitas non est aliud secundum substantiam a tempore.

Argumentum 3

Praeterea, sicut mensura primi motus est mensura omnium motuum, ut dicitur in IV physic., ita videtur quod mensura primi esse sit mensura omnis esse. Sed aeternitas est mensura primi esse, quod est esse divinum. Ergo aeternitas est mensura omnis esse.

Sed esse rerum corruptibilium mensuratur tempore.

Ergo tempus vel est aeternitas, vel aliquid aeternitatis.

Sed Contra

Sed contra est quod aeternitas est tota simul, in tempore autem est prius et posterius. Ergo tempus et aeternitas non sunt idem.

Corpus

Respondeo dicendum quod manifestum est tempus et aeternitatem non esse idem.

Sed huius diversitatis rationem quidam assignaverunt ex hoc quod aeternitas caret principio et fine, tempus autem habet principium et finem. Sed haec est differentia per accidens, et non per se.

Quia dato quod tempus semper fuerit et semper futurum sit, secundum positionem eorum qui motum caeli ponunt sempiternum, adhuc remanebit differentia inter aeternitatem et tempus, ut dicit boetius in libro de consolat., ex hoc quod aeternitas est tota simul, quod tempori non convenit, quia aeternitas est mensura esse permanentis, tempus vero est mensura motus. Si tamen praedicta differentia attendatur quantum ad mensurata, et non quantum ad mensuras, sic habet aliquam rationem, quia solum illud mensuratur tempore, quod habet principium et finem in tempore, ut dicitur in IV physic..

Unde si motus caeli semper duraret, tempus non mensuraret ipsum secundum suam totam durationem, cum infinitum non sit mensurabile; sed mensuraret quamlibet circulationem, quae habet principium et finem in tempore.

Potest tamen et aliam rationem habere ex parte istarum mensurarum, si accipiatur finis et principium in potentia. Quia etiam dato quod tempus semper duret, tamen possibile est signare in tempore et principium et finem, accipiendo aliquas partes ipsius, sicut dicimus principium et finem diei vel anni, quod non contingit in aeternitate. Sed tamen istae differentiae consequuntur eam quae est per se et primo, differentiam, per hoc quod aeternitas est tota simul, non autem tempus.

Ad 1

Ad primum ergo dicendum quod ratio illa procederet, si tempus et aeternitas essent mensurae unius generis, quod patet esse falsum, ex his quorum est tempus et aeternitas mensura.

Ad 2

Ad secundum dicendum quod nunc temporis est idem subiecto in toto tempore, sed differens ratione, eo quod, sicut tempus respondet motui, ita nunc temporis respondet mobili; mobile autem est idem subiecto in toto decursu temporis, sed differens ratione, inquantum est hic et ibi. Et ista alternatio est motus. Similiter fluxus ipsius nunc, secundum quod alternatur ratione, est tempus.

Aeternitas autem manet eadem et subiecto et ratione.

Unde aeternitas non est idem quod nunc temporis.

Ad 3

Ad tertium dicendum quod, sicut aeternitas est propria mensura ipsius esse, ita tempus est propria mensura motus. Unde secundum quod aliquod esse recedit a permanentia essendi et subditur transmutationi, secundum hoc recedit ab aeternitate et subditur tempori. Esse ergo rerum corruptibilium, quia est transmutabile, non mensuratur aeternitate, sed tempore. Tempus enim mensurat non solum quae transmutantur in actu, sed quae sunt transmutabilia.

Unde non solum mensurat motum, sed etiam quietem; quae est eius quod natum est moveri, et non movetur.