(CG 2/32 & 35) Reasons alleged for the Eternity of the World on the part?... (Book 2: God The Origin of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

Arg. 1. There are creatures in which there is no potentiality of not being (see Chap. XXX): it is impossible for them not to be, and therefore they always must be.

Reply (Chap. XXXVI). The necessity of such creatures being is only a relative necessity, as shown above (Chap. XXX): it does not involve the creature's always having been: it does not follow upon its substance: but when the creature is already established in being, this necessity involves the impossibility of its not-being.

Arg. 3. Every change must either go on everlastingly, or have some other change preceding it. But change always has been: therefore also changeable things: therefore creatures.

Reply. It has already been shown (Chapp. XII, XVII) that without any change in God, the agent, He may act to the production of a new thing, that has not always been. But if a new thing may be produced by Him, He may also originate a process of change.

Arg. 5. If time is perpetual, motion must be perpetual, time being the record of motion. But time must be perpetual: for time is inconceivable without a present instant, as a line is inconceivable without a point: now a present instant is always inconceivable without the ending of a past and the beginning of a future instant; and thus every given present instant has before it a time preceding and after it a time succeeding, and so there can be no first or last time. It follows that created substances in motion have been from eternity.

Reply. This argument rather supposes than proves the eternity of motion. The reason why the same instant is the beginning of the future and the end of the past is because any given phase of motion is the beginning and end of different phases. There is no showing that every instant must be of this character, unless it be assumed that every given phase of time comes between motion going before and motion following after, which is tantamount to assuming the perpetuity of motion. Assuming on the contrary that motion is not perpetual, one may say that the first instant of time is the beginning of the future, and not the end of any past instant. Even in any particular case of motion we may mark a phase which is the beginning only of movement and not the end of any: otherwise every particular case of motion would be perpetual, which is impossible.

Arg. 6. If time has not always been, we may mark a non-existence of time prior to its being. In like manner, if it is not always to be, we may mark a non-existence of it subsequent to its being. But priority and subsequence in point of duration cannot be unless time is; and at that rate time must have been before it was, and shall be after it has ceased, which is absurd. Time then must be eternal. But time is an accident, and cannot be without a subject. But the subject of it is not God, who is above time and beyond motion (B. I, Chapp. XIII, XV). The only alternative left is that some created substance must be eternal.

Reply. There is nothing in this argument to evince that the very supposition of time not being supposes that time is (read, Si ponitur tempus non esse, ponatur esse). For when we speak of something prior to the being of time, we do not thereby assert any real part of time, but only an imaginary part. When we say, 'Time has being after not being', we mean that there was no instant of time before this present marked instant: as when we say that there is nothing above the stellar universe, we do not mean that there is any place beyond the stellar universe, which may be spoken of as 'above' it, but that above it there is no 'place' at all.