LECTIO FOURTEEN

             § 399. Having distinguished the proper sense-objects from the common, and from those that are sensible incidentally, the Philosopher now treats of the proper object of each sense: first of the proper object of sight; then, at 'Now let us start', of that of hearing; then, at 'It is not so easy', of that of smell; then, at 'The tasteable', of that of taste; and lastly, at 'The same reasoning holds', of that of touch.

             As to sight, he discusses, first, its object, and then, at 'At present what is clear', how this object comes to be seen. Touching the object of sight, he does two things. First, he determines what is the visible, dividing it into two. Secondly, he deals with either visible, at 'For the visible is colour'. He says then, first, that, the proper sense-object being that which each sense perceives of itself exclusively, the sense-object of which the special recipient is sight is the visible. Now in the visible two things are included; for both colour is a visible, and also something else, which can be described in speech, but has no proper name; which visible belongs to things which can be seen by night, such as glow-worms and certain fungi on oak-trees and the like, concerning which the course of this treatise will inform us more clearly as we gain a deeper understanding of the visible; but we have to start from colour which is the more obvious visible.

             § 400. Then, at 'For the visible', he begins to define both objects of sight: first colour and then, at 'Not all visible things', that of which he says that it has no proper name. As to colour he does two things: first, he shows what colour has to do with visibility; secondly, at 'There is, accordingly, something transparent' he settles what is required for colour to be seen.

             First of all, then, he says that, colour being visible, it is visible of itself, for colour as such is essentially visible.

             § 401. 'Essentially' is said in two ways. In one way, when the predicate of a proposition falls within the definition of the subject, e.g. 'man is an animal'; for animal enters into the definition of man. And since that which falls within the definition of anything is in some way the cause of it, in cases such as these the predicate is said to be the cause of the subject. In another way, on the contrary, when the subject of the proposition falls within the definition of the predicate, as when it is said that a nose is snub, or a number is even; for snubness is nothing but a quality of a nose, and evenness of a number which can be halved; and in these cases the subject is a cause of the predicate.

             § 402. Now colour is essentially visible in this second manner, not in the first; for visibility is a quality, as being snub is a quality of a nose. And this is why he says that colour is visible 'essentially', but 'not by definition'; that is to say, not because visibility is placed in its definition, but because it possesses of itself the reason why it should be visible, as a subject possesses in itself the reason for its own peculiar qualities.

             § 403. Which he proves by this, that every colour as such is able to affect what is actually diaphanous. The diaphanous is the same as the transparent (e.g. air or water), and colour has it in its nature to actualise further an actual transparency. And from this, that it affects the actually transparent, it is visible; whence it follows that colour is of its nature visible. And since the transparent is brought to its act only by light, it follows that colour is not visible without light. And therefore before explaining how colour is seen, we must discuss light.

             § 404. Then, at 'There is, accordingly', he discusses those things without which colour cannot be seen, namely the transparent and light; and this in three sections. First, he explains the transparent. Secondly, at 'Light is', he treats of the transparent's actuality, i.e. light. Thirdly, he shows how the transparent is receptive of colour, at 'Now that only can receive colour'.

             To begin with, therefore, he says that if colour is that which of its nature affects the transparent, the latter must be, and in fact is, that which has no intrinsic colour to make it visible of itself, but is receptive of colour from without in a way which renders it somehow visible. Examples of the transparent are air and water and many solid bodies, such as certain jewels and glass. Now, whereas other accidents pertaining to the elements or to bodies constituted from them, are in these bodies on account of the nature of those elements (such as heat and cold, weight and lightness, etc.), transparency does not belong to the nature of air or water as such, but is consequent upon some quality common, not only to air and water, which are corruptible bodies, but also to the celestial bodies, which are perpetual and incorruptible. For at least some of the celestial bodies are manifestly transparent. We should not be able to see the fixed stars of the eighth sphere unless the lower spheres of the planets were transparent or diaphanous. Hence it is evident that to be transparent is not a property consequent on the nature of air or water, but of some more generic nature, in which the cause of transparency is to be found, as we shall see later.

             § 405. Next, at 'Light etc.', he explains light, first stating the truth, then dismissing an error. He says, to begin with, that light is the act of the transparent as such. For it is evident that neither air nor water nor anything of that sort is actually transparent unless it is luminous. Of itself the transparent is in potency to both light and darkness (the latter being a privation of light) as primary matter is in potency both to form and the privation of form. Now light is to the transparent as colour is to a body of definite dimensions: each is the act and form of that which receives it. And on this account he says that light is the colour, as it were, of the transparent, in virtue of which the transparent is made actually so by some light-giving body, such as fire, or anything else of that kind, or by a celestial body. For to be full of light and to communicate it is common to fire and to celestial bodies, just as to be diaphanous is common to air and water and the celestial bodies.

             § 406. Then, at 'We have then indicated', he rejects a false opinion on light; and this in two stages. First, he shows that light is not a body; then he refutes an objection brought against the arguments which prove that light is not a body, at 'Empedocles . . . was wrong'. As to the first point he does three things.

             (a) He states his own view, saying that, once it is clear what the transparent is, and what light is, it is evident that light is neither fire (as some have said, positing three kinds of fire, the combustible, and flame, and light); nor a body at all, or anything flowing from a body, as Democritus supposed, asserting that light consisted of atomic particles emanating from luminous bodies. If there were these emanations from bodies, they would themselves be bodies, or something corporeal, and light would thus be nothing other than fire, or something material of that sort, present in the diaphanous; which is the same as to say that light is a body or an emanation from a body.

             § 407. (b) At 'For it is impossible', he proves his own hypothesis thus. It is impossible for two bodies to be in one place at one time. If therefore light were a body, it could not co-exist with a diaphanous body; but this is false; therefore light is not a body.

             § 408. (c) At 'Light seems', he shows that light does co-exist with the diaphanous body. For contraries exist in one and the same subject. But light and darkness are contraries in the manner in which a quality and its privation are contraries, as is stated in the Metaphysics, Book X. Obviously, darkness is a privation of this quality, i.e. of light in the diaphanous body--which is therefore the subject of darkness. Hence too, the presence of this quality is light. Therefore light co-exists with the diaphanum.

             § 409. Then at 'Empedocles . . . was wrong', he refutes an answer to one argument which might be urged against those who hold that light is a body. For it is possible to argue thus against them: if light were a body, illumination ought to be a local motion of light passing through the transparent; but no local movement of any body can be sudden or instantaneous; therefore, illumination would be, not instantaneous but successive, according to this view.

             § 410. Of which the contrary is a fact of experience; for in the very instant in which a luminous body becomes present, the transparent is illuminated all at once, not part after part. So Empedocles, and all others of the same opinion, erred in saying that light was borne along by local motion, as a body is; and that it spread out successively through space, which is the medium between the earth and its envelope, i.e. the sky; and that this successive motion escapes our observation, so that the whole of space seems to us to be illuminated simultaneously.

             § 411. For this assertion is irrational. The illumination of the transparent simply and solely presupposes the placing of a luminous body over against the one illumined, with no intervening obstacle.

             § 412. Again, it contradicts appearances. One might indeed allow that successive local motion over a small space could escape our notice; but that a successive movement of light from the eastern to the western horizon should escape our notice is so great an improbability as to appear quite impossible.

             § 413. But as the subject matter under discussion is threefold, i.e. the nature of light, and of transparency, and the necessity of light for seeing, we must take these three questions one by one.

             On the nature of light various opinions have been held. Some, as we have seen, held that light was a body; being led to this by certain expressions used in speaking of light. For instance, we are accustomed to say that a ray 'passes through' the air, that it is 'thrown back', that rays 'intersect', and so forth; which all seem to imply something corporeal.

             § 414. But this theory is groundless, as the arguments here adduced of Aristotle show, to which others might easily be added. Thus it is hard to see how a body could be suddenly multiplied over the whole hemisphere, or come into existence or vanish, as light does; nor how the mere intervention of an opaque body should extinguish light in any part of a transparent body if light itself were a body. To speak of the motion or rebounding of light is to use metaphors, as when we speak of heat 'proceeding into' things that are being heated or being 'thrown back' when it meets an obstacle.

             § 415. Then there are those who maintain, on the contrary, that light is spiritual in nature. Otherwise, they say, why should we use the term 'light' in speaking of intellectual things? For we say that intellectual things possess a certain intelligible 'light'. But this also is inadmissible.

             § 416. For it is impossible that any spiritual or intelligible nature should fall within the apprehension of the senses; whose power, being essentially embodied, cannot acquire knowledge of any but bodily things. But if anyone should say that there is a spiritual 'light' other than the light that is sense-perceived, we need not quarrel with him; so long as he admits that the light which is sense-perceived is not spiritual in nature. For there is no reason why quite different things should not have the same name.

             § 417. The reason, in fact, why we employ 'light' and other words referring to vision in matters concerning the intellect is that the sense of sight has a special dignity; it is more spiritual and more subtle than any other sense. This is evident in two ways. First, from the object of sight. For objects fall under sight in virtue of properties which earthly bodies have in common with the heavenly bodies. On the other hand, touch is receptive of properties which are proper to the elements (such as heat and cold and the like); and taste and smell perceive properties that pertain to compound bodies, according as these are variously compounded of heat and cold, moisture and dryness; sound, again, is due to local movement which, indeed, is also common to earthly and heavenly bodies, but which, in the case of the cause of sound, is a different kind of movement from that of the heavenly bodies, according to the opinion of Aristotle. Hence, from the very nature of the object it would appear that sight is the highest of the senses; with hearing nearest to it, and the others still more remote from its dignity.

             § 418. The same point will appear if we consider the way in which the sense of sight is exercised. In the other senses what is spiritual in their exercise is always accompanied by a material change. I mean by 'material change' what happens when a quality is received by a subject according to the material mode of the subject's own existence, as e.g. when anything is cooled, or heated, or moved about in space; whereas by a 'spiritual change' I mean, here, what happens when the likeness of an object is received in the sense-organ, or in the medium between object and organ, as a form causing knowledge, and not merely as a form in matter. For there is a difference between the mode of being which a sensible form has in the senses and that which it has in the thing sensed. Now in the case of touching and tasting (which is a kind of touching) it is clear that a material change occurs: the organ itself grows hot or cold by contact with a hot or cold object; there is not merely a spiritual change. So too the exercise of smell involves a sort of vaporous exhalation; and that of sound involves movement in space. But seeing involves only a spiritual change--hence its maximum spirituality; with hearing as the next in this order. These two senses are therefore the most spiritual, and are the only ones under our control. Hence the use we make of what refers to them--and especially of what refers to sight--in speaking of intellectual objects and operations.

             § 419. Then again some have simply identified light with the manifestation of colour. But this is patently untrue in the case of things that shine by night, their colour, nevertheless, remaining obscure.

             § 420. Others, on the other hand, have said that light was the substantial form of the sun, and that the brightness proceeding therefrom (in the form of colours in the air) had the sort of being that belongs to objects causing knowledge as such. But both these propositions are false. The former, because no substantial form is in and of itself an object of sense perception; it can only be intellectually apprehended. And if it is said that what the sense sees in the sun is not light itself but the splendour of light, we need not dispute about names, provided only it be granted that what we call light, i.e. the sight-perceived thing, is not a substantial form. And the latter proposition too is false; because whatever simply has the being of a thing causing knowledge does not, as such, cause material change; but the rays from the heavenly bodies do in fact materially affect all things on earth. Hence our own conclusion is that, just as the corporeal elements have certain active qualities through which they affect things materially, so light is the active quality of the heavenly bodies; by their light these bodies are active; and this light is in the third species of quality, like heat.

             § 421. But it differs from heat in this: that light is a quality of the primary change-effecting body, which has no contrary: therefore light has no contrary: whereas there is a contrary to heat. And because there is no positive contrary to light, there is no place for a contrary disposition in its recipient: therefore, too, its matter, i.e. the transparent body, is always as such immediately disposed to its form. That is why illumination occurs instantaneously, whereas what can become hot only becomes so by degrees. Now this participation or effect of light in a diaphanum is called 'luminosity'. And if it comes about in a direct line to the lightened body, it is called a 'ray'; but if it is caused by the reflection of a ray upon a light-receiving body, it is called 'splendour'. But luminosity is the common name for every effect of light in the diaphanum.

             § 422. So much being admitted as to the nature of light, we can easily understand why certain bodies are always actually lucent, whilst others are diaphanous, and others opaque. Because light is a quality of the primary change-effecting body, which is the most perfect and least material of bodies, those among other bodies which are the most formal and the most mobile to actualisation are always actually lucent; and the next in this order are diaphanous; whilst those that are extremely material, being neither luminous of themselves nor receptive of light, are opaque. One may see this in the elements: fire is lucent by nature, though its light does not appear except in other things. Air and water, being more material, are diaphanous; whilst earth, the most material of all, is opaque.

             § 423. With regard to the third point (the necessity of light for seeing), note that it has been the opinion of some that not merely seeing, but the object of seeing, i.e. colour as such, presupposed the presence of light; that colour as such had no power to affect a transparent medium; that it does this only through light. An indication of this was, they said, that one who stands in the shadow can see what is in the light, but one who stands in the light cannot see what is in shadow. The cause of this fact, they said, lay in a correspondence between sight and its object: as seeing is a single act, so it must bear on an object formally single; which would not be the case if colour were visible of itself--not in virtue of light--and light also were visible of itself.

             § 424. Now this view is clearly contrary to what Aristotle says here, 'and . . . has in itself the cause of being visible'; hence, following his opinion, I say that light is necessary for seeing, not because of colour, in that it actualises colours (which some say are in only potency so long as they are in darkness), but because of the transparent medium which light renders actual, as the text states.

             § 425. And in proof of this, note that every form is, as such, a principle of effects resembling itself. Colour, being a form, has therefore of itself the power to impress its likeness on the medium. But note also that there is this difference between the form with a complete, and the form with an incomplete, power to act, that the former is able not merely to impress its likeness on matter, but even to dispose matter to fit it for this likeness; which is beyond the power of the latter. Now the active power of colour is of the latter sort; for it is, in fact, only a kind of light somehow dimmed by admixture of opaque matter. Hence it lacks the power to render the medium fully disposed to receive colour; but this pure light can do.

             § 426. Whence it is also clear that, as light is, in a certain way, the very substance of colour, all visible objects as such share in the same nature; nor does colour require to be made visible by some other, extrinsic, light. That colours in light are visible to one standing in the shade is due to the medium's having been sufficiently illumined.

418b 27-419b 2

SIGHT. HOW COLOUR IS SEEN

             NOW that only can receive colour which has none, as only that which is soundless, can receive sound. What is without colour is the transparent and the invisible, or what is barely seen, being dark. The transparent is precisely of this nature when it is not in act, but in potency. For the same substance is sometimes dark, sometimes light.§§ 427-8

             Not all visible things, however, are visible in light, but only the colour proper to each. There are certain things which are, indeed, not seen in light, but which produce a sensation in darkness, such as those which burn or are luminous. These are not called by any one term. Such are the fungi of certain trees, horn, fish-heads, scales, and eyes. But the colour proper to each of these is not perceived. Why these things are thus seen is matter for another enquiry.§§ 429-30

             At present what is clear is that what is seen in light is colour; [and that] therefore it is not seen without fight. For to be colour is to be able to move the transparent into act; and this act of the transparent is light. A plain proof whereof is that if one places on the sight itself a coloured object, it is not seen. But colour moves the transparent medium (say, air); and the sensitive organ is moved by this extended continuum.§§ 431-2

 Democritus put forward the erroneous opinion that if the medium were a vacuum, perception would be everywhere exact, even of an ant in the sky. This is, however, impossible; for only when the sensitive faculty is affected does vision occur. This cannot, however, be effected by the colour seen, in itself. It must therefore be due to the medium. If there were a vacuum, a thing, so far from being perceived clearly, would not be seen at all. We have stated, then, why it is necessary that colour be seen in light.§§ 433-5

             But fire is seen in both darkness and light: necessarily, for the transparent is made light by it.§ 436

             The same account holds for both sound and smell. No sensation is produced when either of these touches the organ: but a medium is affected by sound and smell, and the sense organ of one or the other sense by the medium. But if one places an object that sounds or smells upon the sense-organ itself, no sensation occurs. The same holds good of touch and taste, although this is not obvious. The reason for this will be made clear later.§ 437

             The medium of sound is air; that of smell has no special name. For as there is a common quality for colour, to wit, the transparent, in air and water, so there is a common quality in them for smell. For it seems that aquatic animals possess a sense of smell. But man, and whatever living things breathe, are unable to smell except when breathing. The cause of this will be dealt with later.§ 438