Lecture 15 — Whether there is a midpoint between the sensible object and the sense

But someone will raise an objection: do either the sensible objects or the movements caused by the sensible objects—however sensation occurs—first reach a midpoint when they act? Odor and sound seem to do this: for one who is closer senses odor earlier, and sound arrives after the striking. Is it the same, then, in the case of a visible object and light?

Empedocles says that light from the sun in this way arrives at a midpoint before arriving at the sense of sight or the earth.

It might be thought reasonable that this is what happens. For what is moved is moved from something to something, and so there is necessarily a time in which it is moved from one to the other. But every length of time is divisible. Thus there was a time when the ray was not seen, but was still being brought in the midpoint.

Everything simultaneously hears and has heard, and in general senses and has sensed. There is no generation of sensations; they have no coming into being. Nevertheless sound is not already at the sense of hearing when the blow is struck. The reconfiguration of letters when they are carried through a midpoint makes this clear: people apparently have not heard what was said, because the air carried is reconfigured in being carried. It is thus in the case of color and light: for it is not the case that this sees and that is seen if they are related in just any way. This is how it is with things that are equal: neither has to be in a particular place, since when things are made equal, it makes no difference if they are either near to or far from one another.

On the other hand, it is reasonable for this to happen in the case of sound and odor. For air and water are indeed in a way continua, but the movements of both are divisible. For this reason too, it is the same thing that the first person and the last person hears and smells, but in a way it is not. But there seems to some to be an objection also about this. For some say that it is impossible for each person, by means of what is different, to hear and see and smell the same thing. For it is not possible for many who stand apart from one another to hear and smell the same thing, that is, for it to be cut off from itself. On the other hand, all sense the original mover—for example the quince, or incense, or fire—which is numerically one and the same. What is individual is numerically different, but specifically the same. Therefore many simultaneously see and smell and hear.

But these things are not bodies, but an affection and a movement. For otherwise this would not happen. Nor are they independent of body.

But the account of light is different, for there is light through some one single being, but there is not a movement.

In general, alteration and transfer are not alike. For it is reasonable that transfers should first reach a midpoint; and sound seems to be a movement of transfer. But what is altered is not like this.

For it can happen that a whole is altered all at once, not half of it first, for example when a whole body of water is solidified all at once.

Nevertheless, if what is heated or solidified is large, what is contiguous is affected by the contiguous. But the first is necessarily altered, all at once and instantaneously, by the very thing that is acting. And tasting would be like odor if we existed in moisture, and sensed it from a distance before touching it.

But it is reasonable, where the sensitive part has a midpoint, that not all of it be affected all at once, except in the case of light, because of what was said above, and, for the same reason, in the case of seeing: for light causes seeing.

224. But someone will raise an objection (446a20). After the Philosopher has followed up the first question, which concerns sensible things themselves, here he proceeds to the second question, which concerns the alteration of the sense power by sensible objects. On this point, he does three things.

First, he raises the question.

Second, he argues it, where he says Empedocles says (446a26; [226]).

Third, he solves it, where he says on the other hand (446b13; [236]).

On the first point it must be considered that, as was established above, (Cf. cc. 2–3, nn. 46–51; ch. 4, n. 60; ch. 7, n. 105.) some held that a sense power is changed by sensible things by way of an emanation, so that it is the sensible things themselves—that is, the emanations from them—that reach the sense power. But Aristotle himself held that the sensible things change the midpoint by way of a certain alteration, so that it is the changes of this kind that reach the sense power.

225. Therefore there is a question, however sensation occurs: whether the sensible objects themselves—according to the opinion of others—or alterations caused by the sensible objects—according to Aristotle’s own opinion—first reach a midpoint before reaching the sense power. There is no difficulty about this in the case of hearing and smell, for it is clear that one senses odor earlier from nearby, and similarly that sound reaches hearing after the striking of the blow that causes sound occurs, as one who sees the blow from a distance can clearly perceive. And it is clear that this question has no place in the case of taste and touch, because they do not perceive through an external medium. Hence the difficulty seems to be about sight alone, that is, whether a visible object and the light that causes seeing first reach a midpoint before reaching the sense power or any terminus.

226. Then, when he says Empedocles says (446a26), he raises an objection concerning the question asked.

First, he argues on the false side of the question.

Second, he eliminates a false solution where he says everything simultaneously hears (446b2; [228]).

He argues in response to the question first by the authority of Empedocles, who said that light proceeding from the sun first reaches a midpoint before reaching the sense of sight that sees the light, or the earth that is seen by means of the light, and beyond which the sun’s ray does not proceed.

He touched on this question in On the Soul 2, but disproved the opinion as follows: in such a great distance as there is from the rising sun to ourselves, it is impossible for a temporal succession to escape our notice. (Aristotle, On the Soul 2.7, 418b20–26; cf. Aquinas, Commentary on Aristotle’s On the Soul, bk. 2, lect. 14, nn. 409–12.)

227. Second, where he says it might be thought reasonable (446a28), he argues the same point by reason. He says that it seems reasonable for this to happen—that is, for the visible object or light to first reach a midpoint before reaching the sense of sight. For there seems to be some movement of the visible object itself, or of the light, in coming to the sense of sight. But everything that is moved is moved from something to something, in such a way that before, it is at the terminus from which it is moved, and after, it is at the terminus to which it is moved: otherwise, if it were simultaneously at both termini, it would not be moved from one to the other.

But before and after in movement are counted by time. Therefore there is necessarily some time in which the visible object, or the light, is moved from the visible or illuminating body to the sense of sight. But every length of time is divisible, as was proved in Physics 6. (Aristotle, Physics 6.1, 232a18–22; 6.2, 232b20–233a10; 6.4, 235a11–12; cf. Aquinas, Commentary on Physics, bk. 6, lect. 3.) Therefore if we take the midpoint of the time in question, at that point the ray of light, or the visible object itself, has not yet reached the sense of sight, but is still being moved through the midpoint, because the magnitude through which something is moved must be divided according to division of time, as was proved in Physics 6. (Aristotle, Physics 6.2, 233a10–17.)

228. Then, when he says everything simultaneously hears (446b2), he eliminates an inadequate response.

Someone could think that sensible objects do not first reach a midpoint before reaching the sense power because sense perceives a sensible object all at once, without successiveness. Thus, in hearing, hearing does not come before having been heard in the way that, in what is successive, being moved does come before having been moved: rather, when someone is hearing, he simultaneously already has heard, because the whole act of hearing is completed in an instant. And this is universally true of every sense, that is, that it simultaneously senses and has sensed something. This is so because there is no generation of sensations; they have no coming into being.

229. We say that there is a “generation” of things that have a being (esse) which is reached by a successive or gradual movement: whether the terminus of the successive movement is the very form of the things, as when a white thing is said to be generated because a thing reaches whiteness through successive alteration; or whether the terminus of the successive movement is a disposition to their form, as fire and water are said to be generated because dispositions to their forms—that is, elemental qualities—are acquired through successive alteration.

230. But those things begin to be without being generated or coming into being that are not—either of themselves or with respect to any preceding dispositions in them—caused through successive movement. For example, “to the right” is caused in a thing not by any successive movement preexisting in it, but by something else having been made “to the left” of it. Likewise, air begins to be illuminated not by any movement preexisting in it, but at the presence of an illuminating body. And likewise, a sense power begins to sense not by any movement preexisting in it, but at the requisite placing of a sensible object before it. Thus one simultaneously senses and already has sensed.

231. Nevertheless, it is not for this reason necessary that sensible objects, or movements of sensible objects, reach a sense power without succession. For it is clear that one simultaneously is hearing and has heard, and nevertheless sound does not reach hearing immediately when the blow that causes the sound is struck.

232. This is made clear by the reconfiguration of letters that occurs when someone’s speech is heard from a distance, indicating that the sound of the voice formed into letters is being carried successively through the midpoint. It is for this reason that those who hear the sound apparently have not by hearing distinguished the letters pronounced: because the air moved in the midpoint is reconfigured, as if losing the impression made by the one who first caused the sound.

233. This sometimes happens because of some other alteration in the air, for example when, because many are speaking, it is impossible to make out what one of them is saying, for the movements impede one another. But sometimes it happens because of distance: for as the action of heating is weakened in what is farther away, so also does the alteration of the air by the one who first produces the sound, and as a result the sound of the speech may reach those who are near the speaker perfectly, with the requisite articulation of letters, but reach those who are farther away with some confusion.

234. The case of color and light (446b9), then, seems to be similar, for color and light also cannot be seen by being positioned in just any way: rather, a determinate distance is required. For just as utterances are heard by those at a distance without distinction of the letters, so also bodies are seen by those at a distance without distinction of the arrangement of individual parts.

235. The relation between the sense of sight and the visible object is not like the relation of equality: for no determinate location is required in order for things to be equal; rather, however their location varies, they always remain equal in the same way, and it makes no difference whether they are near or far. Therefore it seems that just as the reconfiguration of letters makes it clear that sound reaches the sense of hearing by succession, although once it has reached hearing, it is heard all at once; so the incomplete vision of remote visible objects seems to indicate that color and light reach the sense of sight by succession, although they are seen all at once.

236. Then, when he says on the other hand (446b13), he presents the true solution, showing the difference between sight and the other two senses that perceive through external media, namely hearing and smell. This is divided into two parts.

First, he gives the difference of sight from hearing and smell.

Second, he concludes to his proposal, where he says but it is reasonable (447a8; [252]).

The first part is divided into two according to the two differences he gives.

The second begins where he says in general, alteration and transfer (446b28; [244]).

Accordingly, he first says that it is reasonable for this to happen in the case of sound and odor, namely that they reach the sense power by succession. He gives as the reason for this the fact that air and water, which are the media by which these objects are brought to the sense power, are indeed, in their substance, continua, and yet movements distinct from one another can take place in them. This can happen because of the easy divisibility of air and water, which, as the Philosopher shows in Physics 7, is evident in the movement of throwing something, where there are many movements, many movers, and many things moved. (Aristotle, Physics 7.2, 244a3–245b1; cf. Aquinas, Commentary on Physics, bk. 7, lect. 3.) For one part of the air is moved by another, and thus there are different movements succeeding one another, because a part of the air that has been moved still remains a mover after it ceases to be moved. Thus the movements of the parts of the air are not all simultaneous; rather they succeed one another, as is shown in Physics 8. (Aristotle, Physics 8.10, 266b27–267a12; cf. Aquinas, Commentary on Physics, bk. 8, lect. 22.)

237. This is also evident in the case of sound, which is caused by a striking of air, but not in such a way that the whole of the air in between is moved by what strikes it in one movement. Rather, there are many movements succeeding one another, for one part, having been moved first, then moves another. Thus in a way it is the same thing that the first person hears—the one who is close to the striking that causes the sound—and that the last person hears—the one who is at a distance. But in a way it is not the same thing.

238. According to some, there seems to be (446b17) a difficulty on this point: some say that, since different people sense by means of different organs, it is impossible that they sense the same thing. Now this is true if it refers to what proximately moves the sense power, because the senses of different people are immediately altered by the different parts of the midpoint that are close to them, and so what one senses is “cut off,” and distinct from, what another senses. But if what is understood is what first moves the midpoint, then all sense one and the same thing. For instance everyone, whether near or far, hears the sound of the one blow; likewise, everyone smells the one odorous body, for example the quince, or the incense burning in fire. What reaches each one individually one is numerically different, but is specifically the same, because all these alterations are caused by the same form, the form of what first activates them. Hence many simultaneously see and smell and hear the same sensible object by the different alterations that reach them.

239. But these things (446b25) that reach the senses of each individual are not bodies emanating from a sensible body, as some held: (Cf. cc. 2–3, nn. 46–51; ch. 4, n. 60; ch. 7, n. 105; ch. 15, n. 224.) rather, every individual one of them is a movement and an affection of a midpoint that has been altered by the action of a sensible object. For if they were different bodies that reached different individuals by emanation, then this—that is, everyone sensing the same thing—would not happen, but each would perceive only the body that reached him. And although they are not bodies, nevertheless they are not independent of body, or of the midpoint that is as it were affected and moved, or of the sensible object that is as it were what first moves and acts.

It is clear, then, from what has been said that sound reaches the sense of hearing through many successive movements of parts. And it is similar in the case of odor, except that alteration by odor occurs through alteration of a midpoint, but alteration by sound through local movement.

240. But the account of light is different (446b27). For light does not reach the sense of sight through many movements succeeding one another in different parts of the midpoint, but through one single being (esse)—that is, by the whole midpoint being moved, as one moveable thing, in one movement, by an illuminating body. In this case there is not one movement succeeding another, as there was said to be in the case of odor and sound.

241. The reason for this difference is that what is received in something as in its proper and natural subject can remain in it and be a principle of action, but what is received in something only as an adventitious quality can neither remain in it nor be a principle of action. Now because substantial forms are the principles of qualities and of all accidents, that quality is received in a subject according to its proper and natural being which disposes the subject to the natural form of which it is receptive. For instance, water, by reason of its matter, is receptive of the substantial form of fire, which is the principle of heat; and so heat is received in water as disposing it to the form of fire, and when fire is removed, the water still remains hot and capable of heating.

242. Likewise odor is received in air and water, and sound in air, both according to their proper and natural being, and according as air and water are altered by enchymous dryness, or air by the striking of a body. Thus, when the striking ceases, sound remains in the air; and when the odor-causing body is removed, odor is still perceived in the air. The reason is that the part of the air that has been changed so as to take on a sound or odor can likewise change another part, and thus different movements are produced that succeed one another.

243. But the transparent is not receptive of the substantial form of the illuminating body, for instance the sun, which is the first “root” of light, and neither is it disposed by reception of light to any substantial form. Hence light is received in the transparent as an adventitious quality that neither remains when the illuminating body is absent, nor is able to be a principle of action on something else. Hence one part of the air is not illuminated by another, but the whole of the air is illuminated by what first illuminates it, however far the power of the illuminating body is able to reach. Thus, there is one thing illuminated, and one illumination of the whole midpoint.

244. Then, when he says in general, alteration and transfer (446b28), he shows the second difference. He says that, to speak generally about alteration and “transfer”—that is, change of place—the two are not alike. It is reasonable that changes of place should first reach the midpoint of the magnitude over which the movement takes place before reaching the terminus, since in change of place there is movement from one extreme of a magnitude to its other extreme. Hence, the moveable thing must reach the midpoint of the magnitude at the midpoint of the time. Thus the argument introduced above has a place in the case of change of place. And sound is a consequence of local movement inasmuch as the air is disturbed by the striking that causes the sound all the way to the power of hearing. It is reasonable, then, that sound should reach a midpoint before reaching the sense of hearing.

245. But in the case of what is altered, it is not like this. For the termini of alteration are not extremes of a magnitude, and so the time of the alteration is not necessarily, of itself, commensurate with a magnitude in such a way that at the midpoint of the time, the movement would reach the midpoint of the magnitude, whether the midpoint of the magnitude over which the movement occurs (for this cannot be given in alteration, which is not a movement in a quantity or a “where,” but in a quality), or the midpoint of a magnitude that is itself moved.

For it (447a1) can sometimes happen that the whole of a body is altered all at once, not half of it first, as we see that a whole body of water may be frozen all at once.

In local movement, the time is commensurable with the magnitude over which the movement passes, and is divided according to division of the magnitude, as is proved in Physics 6. (Aristotle, Physics 6.1–2, 232a18–233a17; cf. Aquinas, Commentary on Physics, bk. 6, lect. 3.) Likewise, in alteration, the time is commensurable with the distance between the termini, and so, other things being equal, more time is required for something cold than for something tepid to be made hot.

246. Thus if there are some extremes between which one cannot take a midpoint, transition from one extreme to the other must occur without intermediary. Now a contradiction is an opposition that of itself has no intermediary, as is said in Posterior Analytics 1. (Aristotle, Posterior Analytics 1.2, 72a12–13; cf. Aquinas, Commentary on Posterior Analytics, bk. 1, lect. 5.) And, with the supposition of the receptivity of a subject, the account of privation is the same, for privation is nothing but negation in a subject. Hence all alterations of which the termini are being and non-being, or privation and form, are instantaneous and cannot be successive.

247. For in successive alterations the succession is noted by means of determinate intermediaries with respect to the distance of one contrary from the other.

Within this distance, the whole magnitude of the body over which the power of the first cause of the alteration immediately extends is considered as one single subject that instantaneously, all at once, begins to be moved. But if there is a body that is capable of being altered, but is so large that the power of the first cause of the alteration cannot reach it as a whole, but only part of it, it will follow that after the first part has been altered, it will subsequently alter another one.

And so he says that if it is a large body that is heated or frozen, what is contiguous is necessarily affected by the contiguous—that is, a subsequent part is necessarily altered by the immediately preceding one. But the first part is altered, all at once and instantaneously, by the very thing that first causes the alteration, because here there is not succession from the point of view of magnitude, but only from the point of view of contrary qualities, as was said.

248. It is because of this that odor first reaches a midpoint before reaching the sense power, even though this occurs by alteration without local movement. For an odorous body cannot alter the whole medium all at once; rather, it alters one part, which alters another, and so the alteration reaches the sense of smell by succession, through several movements, as was said above. (Cf. ch. 13, nn. 200–201.) And it would be the same in the case of taste as it is in the case of smell if we lived in watery moisture, which alone is receptive of flavor, as we now live in air, which is receptive of odor; and if, again, flavor could be sensed by alteration of the medium from a distance before we touched a flavored body, as is the case with smell.

249. Now what is said here seems to be contrary to the argument by which the Philosopher proves in Physics 6 that everything that is moved is divisible, because part of it is at the terminus a quo and part at the terminus ad quem. (Aristotle, Physics 6.4, 234b10–20.; cf. Aquinas, Commentary on Physics, bk. 6, lect. 5, lect. 12.) Thus it can be seen that while something is being changed from white to black, when one part of it is white, another is black. Thus it cannot be that the whole is altered all at once, but rather part after part.

250. But some say that the intention of the Philosopher there is to show not that one part of a moveable thing is at the terminus a quo and another at the terminus ad quem, but that the moveable thing is at one part of the terminus a quo and at another part of the terminus ad quem. And so in alteration it is necessary not that one part of the moveable thing be altered before another, but that the whole moveable thing that is altered, for instance from white to black, have a part of whiteness and a part of blackness.

But this does not agree with Aristotle’s intention, because this would not directly prove that a moveable thing is divisible, but rather that the termini of movement are somehow divisible. Nor again does it agree with the words Aristotle uses, as is clear to anyone who carefully looks at his text, where the passage clearly refers to the parts of the moveable thing.

251. Thus a different explanation has to be given, namely that the demonstration is understood with reference to local movement, which is truly and of itself continuous. For in Physics 6 Aristotle treats of movement under the aspect of the continuous. But movements of growth and alteration, as was said in Physics 8, are not simply continuous. (Aristotle, Physics 8.7, 261a31–b3; cf. Aquinas, Commentary on Physics, bk. 8, lect. 15.) Hence Aristotle’s remark is not universally true of all alteration, but only of alteration inasmuch as it gets continuity from a moveable thing in which one part alters another. But a moveable thing that the power of the first cause of the alteration reaches all at once, as a whole, is something indivisible inasmuch as it is altered all at once.

252. Then, when he says but it is reasonable (447a8), he concludes from the foregoing to the principal intention. He says that it is reasonable, in the case of the senses for which there is a midpoint between the sensible object and the organ of sensing, that the whole midpoint be affected and moved not all at once, but by succession, except in the case of light, and this because of what has been said:

first, because illumination does not, as Empedocles held, occur by local movement, as the spread of sound does, but by alteration;

second, because in illumination there are not several movements, as there were said to be in the case of odor, but only one.

To these points we must add a third: that light has no contrary, but darkness is rather opposed to it as simple privation, and so illumination occurs all at once.

And the same must be said about vision, because light causes seeing, and hence the medium is altered by visible objects analogously to the way it is altered by light.