Thomas Aquinas, Commentary on Aristotle's Physics

First he disproves the positions of others. Secondly, where he says, 'But as time is . . .' (218 b 8), he inquires how time is related to motion, which seems to be most closely related to time.

Concerning the first part he makes two points. First he gives the opinions of others concerning time. Secondly, where he says, 'Yet part, too, of the revolution . . .' (218 b 1), he refutes these opinions.

He says, therefore, first that what time is and what its nature is cannot be settled by means of that which has been handed down from the ancients concerning time. Moreover, the things which they have decided about this matter cannot be reconciled with each other. For some have said that time is the motion of the heavens, and others have said that time is the celestial sphere itself.

566. Next where he says, 'Yet part, too, of the revolution . . .' (218 b 1), he refutes these opinions. First, he refutes the first opinion, and secondly the other one, where he says, 'Those who said . . .' (218 b 5).

Concerning the first opinion he gives two arguments, the first of which is as follows.

If time is a revolution, it would be necessary that part of a revolution is a revolution, since a part of time is time. But part of a revolution is not a revolution. Therefore time is not a revolution.

He gives the second argument where he says, 'Besides, if there were . . .' (218 b 3). The argument is as follows.

Motion is multiplied according to the multitude of mobile bodies. If, therefore, there were many heavens, there would be many revolutions. And thus, if time is a revolution, it would follow that there would simultaneously be many times. This is impossible. For there cannot simultaneously be two parts of time, unless one contains the other, as was said above.

Nevertheless, these men were motivated to hold that time is a revolution because times seem to be repeated in a circle.

567. Next where he says, 'Those who said . . .' (218 b 5), he refutes the second opinion.

He says that it seemed to some that time is the sphere of the heavens because all things are in time and are also in the sphere of the whole, because the heavens contain all things. Hence they wished to conclude that time is the sphere of the heavens.

There are two defects in this argument. First, a thing is not said to be in time and in place univocally. Secondly, they argued in the second figure from two affirmatives. Therefore he says that this position is so stupid that it is not necessary to consider the impossibilities which result from it. For it is clear that all the parts of the sphere exist simultaneously, but all the parts of time do not.

568. Next where he says, 'But as time is . . .' (218 b 8), he inquires how time is related to motion.

First he shows that time is not motion. Secondly, where he says, 'But neither does time . . .' (218 b 22), he shows that there is no time without motion.

Regarding the first part he gives two arguments to show that time is not motion or mutation, which it very much seems to be. Every motion and mutation is only in that which is changed, or else in the place where that which is changed and that which changes are. The first of these applies to substantial, quantitative, and qualitative motion; the second applies to motion in a 'where', which is called local motion. But time is everywhere and among all things. Therefore, time is not motion.

569. He gives the second argument where he says, 'Again, change is always . . .' (218 b 14). The argument is as follows.

Every mutation and motion is either fast or slow. But time is neither fast nor slow. Therefore time is neither motion nor mutation. He proves the middle as follows. Fast and slow are determined by time. For that is called fast which is moved through much space in a short time. Conversely, that is called slow which is moved through a short space in much time. But time is not determined by time, either in its quantity or in its quality. For nothing is the measure of itself. Therefore time is neither fast nor slow. And since he had proposed that mutation is either fast or slow, but no mention of motion was made, he adds that for the present, motion does not differ from mutation. In Book V the difference between them will be explained.

570. Next where he says, 'But neither does time . . .' (218 b 22), he shows that there is no time without motion. When men are not changed in their apprehensions, or if changed, it escapes them, then it does not seem to them that any time has passed. This is clear in regard to those who are fabled to have slept among the Heroes, or the gods, in Sardos, a city in Asia. The souls of the good and the great are called Heroes, and men revered them as gods, such as Hercules and Bacchus and others. Through certain incantations, some were made insensible, and these, they said, slept among the Heroes. For when they had awakened, they said they had seen wonderful things, and they predicted future events. However, when they returned to themselves, they did not perceive the time that had passed while they were so absorbed. For they joined the first instant in which they began to sleep with the later 'now' in which they awoke, as if they were one. They did not perceive the middle time. Therefore, if the 'now' were not different, but were one and the same, there would be no middle time. And thus, when the difference between two 'nows' goes unnoticed, it would not seem that there is a middle time. Therefore when we do not perceive some mutation, time is not thought of, and it seems to a man that he exists in one indivisible 'now'. But we perceive that time comes to be when we sense and we number motion or mutation. It clearly follows that there is no time without motion or mutation.

Lastly he concludes that time is not motion, and there is no time without motion.