LECTURE 17 (219 a 2-b 8)

THE DEFINITION OF TIME IS GIVEN AND EXPLAINED

             571. After the Philosopher has investigated time by means of disputation, he begins here to determine the truth.

             First he determines the truth about time. Secondly, where he says, 'It is also worth considering . . .' (223 a 16), he raises and answers certain difficulties concerning the truth he has determined.

             Concerning the first part he makes two points. First he treats time in itself. Secondly, where he says, 'Time is a measure . . .' (221 a 1), he treats time in comparison with those things which are measured by time.

             Concerning the first part he makes three points. First he explains what time is. Secondly, where he says, 'Just as motion is . . .' (219 b 9), he explains what the 'now' of time is. Thirdly, where he says, 'It is clear, then . . .' (220 a 24), from the given definition of motion he establishes the meanings [ratio] of those things which are said about time.

             Concerning the first part he makes two points. First he gives the definition of time, and secondly he explains it, where he says, 'A proof of this . . .' (219 b 3).

             The first part is divided according to the three parts of the definition of time which he examines. The second part begins where he says, 'But what is moved . . .' (219 a 10), and the third part begins where he says, 'Now we mark them . . .' (219 a 26).

             572. Therefore, he first examines the point that time pertains to motion.

             He says that, since we are asking what time is, we must begin by inquiring how time pertains to motion. It is clear that time does pertain to motion because we sense motion and time together. It happens that sometimes we perceive a passage of time, even if we do not sense any particular, sensible motion, for example, when we are in the dark and do not see the motion of any exterior body. And if we do not suffer any alteration in our bodies from an exterior agent, we will not sense any motion of a sensible body. Nevertheless, if some motion occurs in our soul, for example, a succession of thoughts and images, it immediately seems to us that some time has passed. Thus by perceiving some sort of motion, we perceive time. And conversely, when we perceive time, we also perceive motion. Hence, since time is not motion itself, as was proven, it follows that time pertains to motion.

             573. However, there is a difficulty that arises here concerning the perception of time and motion. If time is consequent upon a sensible motion outside the soul, it follows that he who does not sense that motion will not sense time. However, the contrary of this is held here. But if time is consequent upon a motion of the soul, it would follow that things are not related to time except by the mediation of the soul. And thus time will not be a thing of nature but an intention of the soul, by way of an intention of genus and species. And if time is consequent upon all motion universally, it would follow that there are as many times as there are motions. But this is impossible, because two times are not simultaneous, as was said above.

             574. To answer this it must be known that there is one first motion which is the cause of all other motion. Hence, whatever is mutable in existence is such because of that first motion, which is the motion of the first mobile object. Moreover, whoever perceives any motion, either existing in sensible things or in the soul, perceives a mutable existence, and consequently he perceives the first motion from which time follows. Hence, whoever perceives any motion perceives time, although time is consequent upon only the one first motion by which all other motions are caused and measured. And thus there remains only one time.

             575. Next where he says, 'But what is moved . . .' (219 a 10), he examines the second part of the definition of time. It has been stated that time pertains to motion, that is, it is consequent upon motion. We must ask further how time is consequent upon motion--that is, in respect to before and after.

             Concerning this he makes three points. First he shows how before and after are found in motion. Secondly, where he says, 'The "before" and "after" . . .' (219 a 19), he shows how before and after are related to motion. Thirdly, where he says, 'But we apprehend time . . .' (219 a 22), he shows that time is consequent upon motion in respect to before and after.

             Concerning the first part he makes two points. First he shows that there is continuity in time from motion and magnitude. Secondly, where he says, 'The distinction of . . .' (219 a 13), he shows that there is a before and after in time.

             576. He says, therefore, first that everything which is moved is moved from something to something. But the first of all motions is local motion, which is motion from place to place in respect to some magnitude. But time is consequent upon the first motion. Therefore to investigate time it is necessary to consider motion in respect to place. Hence, since motion in respect to place is motion from something to something in respect to magnitude, and since every magnitude is continuous, then it is necessary that motion is consequent upon magnitude in continuity, that is, since magnitude is continuous, motion is continuous. And consequently time is also continuous. For there seems to be the same amount of time as there is of first motion. But time is not measured by the quantity of just any motion. For the slow is moved through a short space in a long time, and the fast is moved through a long space in a short time. Rather time is consequent upon the quantity of only the first motion.

             577. Next where he says, 'The distinction of . . .' (219 a 13), he shows that the same order is found in before and after. He says that before and after are first in place or in magnitude.

             This is so because magnitude is quantity which has position. But before and after belong to the nature [ratio] of position. Hence, place has a before and after from its very position. And since there is before and after in magnitude, it is necessary that in motion there is a before and after in proportion to the things which are in magnitude and in place. And consequently there is also a before and after in time. For motion and time are so related that one of them always follows upon the other.

             578. Next where he says, 'The "before" and "after" . . .' (219 a 19), he shows how before and after are related to motion.

             He says that the before and after of time and motion, in respect to that which is, are the motion. But in respect to reason [ratio] they are other than motion and are not motion. For according to reason [ratio] motion is the act of that which exists in potency. But before and after are in motion because of the order of the parts of magnitude. Therefore, before and after are the same as motion in subject, but different in reason [ratio]. Hence, since time is consequent upon motion, as was shown above, it must further be asked whether time is consequent upon motion as motion or upon motion insofar as motion has a before and after.

             579. Next where he says, 'But we apprehend time . . .' (219 a 22), he shows that time is consequent upon motion by reason [ratio] of the before and after.

             It was shown that time is consequent upon motion because we know time and motion together. According to this, therefore, time is consequent upon motion according to the knowledge by which time is perceived in motion. But we know time when we distinguish motion by determining a before and after. We say that time passes when we sense a before and after in motion. It follows, therefore, that time is consequent upon motion in respect to before and after.

             580. Next where he says, 'Now we mark them . . .' (219 a 26), he shows what time is, namely, the number of motion. He shows this in the same way, that is, by our knowledge of time and motion.

             It is clear that we determine that there is time when we take two parts of motion with some medium between them. For when we know the diverse extremes of some medium, the soul also says that there are two 'nows', one before, the other after; as if we would say that there is time by numbering the before and after in motion. For time seems to be determined by the 'now' itself. This is supposed for the present. It will be clarified later on.

             Therefore, when we sense one 'now' and do not discern in motion a before and after, or when we discern in motion a before and after but we take the same 'now' as the end of the before and the beginning of the after, then it does not seem that time passes, for there is no motion. But when we take a before and after and number them, then we say that time passes. This is so because time is nothing else than the number of motion in respect to before and after. For we perceive time, as was said, when we number the before and after in motion. Therefore, it is clear that time is not motion, but is consequent upon motion insofar as it is numbered. Hence time is the number of motion.

             If someone objects to the above definition by saying that before and after are determined by time and thus the definition is circular, it must be said that before and after are placed in the definition of time insofar as they are caused in motion by magnitude and not insofar as they are measured by time. Thus Aristotle showed above that before and after are first in magnitude rather than in motion, and in motion rather than in time, so that this objection might be excluded.

             581. Next where he says, 'A proof of this . . .' (219 b 3), he clarifies the above definition in two ways.

             He does this first with an example. We judge something to be more or less by its number. But we judge motion to be more or less by time. Therefore, time is a number.

             Secondly where he says, 'Number, we must note . . .' (219 b 5), he clarifies what was said by distinguishing number. He says that number is twofold. First there is that which is number in act or is that which is numerable, as when we say ten men or ten horses. This is called numbered number, because it is a number which is applied to numbered things. Secondly, there is the number by which we number, that is, number taken absolutely; for example, two, three, four. Time is not a number by which we number. For it would then follow that the number of anything would be time. Rather time is numbered number, because the very number of before and after in motion is time, or also because the very things which are before and after are numbered.

             Therefore, although number is a discrete quantity, nevertheless time is a continuous quantity because of the numbered thing. Thus, ten measures of cloth is a continuum, even though the number ten is a discrete quantity.