Various Works

  the term 'a potential sentient'. There are no separate names for the

  two stages of potentiality; we have pointed out that they are

  different and how they are different. We cannot help using the

  incorrect terms 'being acted upon or altered' of the two transitions

  involved. As we have said, has the power of sensation is potentially

  like what the perceived object is actually; that is, while at the

  beginning of the process of its being acted upon the two interacting

  factors are dissimilar, at the end the one acted upon is assimilated

  to the other and is identical in quality with it.

  6

  In dealing with each of the senses we shall have first to speak of

  the objects which are perceptible by each. The term 'object of

  sense' covers three kinds of objects, two kinds of which are, in our

  language, directly perceptible, while the remaining one is only

  incidentally perceptible. Of the first two kinds one (a) consists of

  what is perceptible by a single sense, the other (b) of what is

  perceptible by any and all of the senses. I call by the name of

  special object of this or that sense that which cannot be perceived by

  any other sense than that one and in respect of which no error is

  possible; in this sense colour is the special object of sight, sound

  of hearing, flavour of taste. Touch, indeed, discriminates more than

  one set of different qualities. Each sense has one kind of object

  which it discerns, and never errs in reporting that what is before

  it is colour or sound (though it may err as to what it is that is

  coloured or where that is, or what it is that is sounding or where

  that is.) Such objects are what we propose to call the special objects

  of this or that sense.

  'Common sensibles' are movement, rest, number, figure, magnitude;

  these are not peculiar to any one sense, but are common to all.

  There are at any rate certain kinds of movement which are

  perceptible both by touch and by sight.

  We speak of an incidental object of sense where e.g. the white

  object which we see is the son of Diares; here because 'being the

  son of Diares' is incidental to the directly visible white patch we

  speak of the son of Diares as being (incidentally) perceived or seen

  by us. Because this is only incidentally an object of sense, it in

  no way as such affects the senses. Of the two former kinds, both of

  which are in their own nature perceptible by sense, the first

  kind-that of special objects of the several senses-constitute the

  objects of sense in the strictest sense of the term and it is to

  them that in the nature of things the structure of each several

  sense is adapted.

  7

  The object of sight is the visible, and what is visible is (a)

  colour and (b) a certain kind of object which can be described in

  words but which has no single name; what we mean by (b) will be

  abundantly clear as we proceed. Whatever is visible is colour and

  colour is what lies upon what is in its own nature visible; 'in its

  own nature' here means not that visibility is involved in the

  definition of what thus underlies colour, but that that substratum

  contains in itself the cause of visibility. Every colour has in it the

  power to set in movement what is actually transparent; that power

  constitutes its very nature. That is why it is not visible except with

  the help of light; it is only in light that the colour of a thing is

  seen. Hence our first task is to explain what light is.

  Now there clearly is something which is transparent, and by

  'transparent' I mean what is visible, and yet not visible in itself,

  but rather owing its visibility to the colour of something else; of

  this character are air, water, and many solid bodies. Neither air

  nor water is transparent because it is air or water; they are

  transparent because each of them has contained in it a certain

  substance which is the same in both and is also found in the eternal

  body which constitutes the uppermost shell of the physical Cosmos.

  Of this substance light is the activity-the activity of what is

  transparent so far forth as it has in it the determinate power of

  becoming transparent; where this power is present, there is also the

  potentiality of the contrary, viz. darkness. Light is as it were the

  proper colour of what is transparent, and exists whenever the

  potentially transparent is excited to actuality by the influence of

  fire or something resembling 'the uppermost body'; for fire too

  contains something which is one and the same with the substance in

  question.

  We have now explained what the transparent is and what light is;

  light is neither fire nor any kind whatsoever of body nor an efflux

  from any kind of body (if it were, it would again itself be a kind

  of body)-it is the presence of fire or something resembling fire in

  what is transparent. It is certainly not a body, for two bodies cannot

  be present in the same place. The opposite of light is darkness;

  darkness is the absence from what is transparent of the

  corresponding positive state above characterized; clearly therefore,

  light is just the presence of that.

  Empedocles (and with him all others who used the same forms of

  expression) was wrong in speaking of light as 'travelling' or being at

  a given moment between the earth and its envelope, its movement

  being unobservable by us; that view is contrary both to the clear

  evidence of argument and to the observed facts; if the distance

  traversed were short, the movement might have been unobservable, but

  where the distance is from extreme East to extreme West, the draught

  upon our powers of belief is too great.

  What is capable of taking on colour is what in itself is colourless,

  as what can take on sound is what is soundless; what is colourless

  includes (a) what is transparent and (b) what is invisible or scarcely

  visible, i.e. what is 'dark'. The latter (b) is the same as what is

  transparent, when it is potentially, not of course when it is actually

  transparent; it is the same substance which is now darkness, now

  light.

  Not everything that is visible depends upon light for its

  visibility. This is only true of the 'proper' colour of things. Some

  objects of sight which in light are invisible, in darkness stimulate

  the sense; that is, things that appear fiery or shining. This class of

  objects has no simple common name, but instances of it are fungi,

  flesh, heads, scales, and eyes of fish. In none of these is what is

  seen their own proper' colour. Why we see these at all is another

  question. At present what is obvious is that what is seen in light

  is always colour. That is why without the help of light colour remains

  invisible. Its being colour at all means precisely its having in it

  the power to set in movement what is already actually transparent,

  and, as we have seen, the actuality of what is transparent is just

  light.

  The following experiment makes the necessity of a medium clear. If

  what has colour is placed in immediate contact with the eye, it cannot

/>   be seen. Colour sets in movement not the sense organ but what is

  transparent, e.g. the air, and that, extending continuously from the

  object to the organ, sets the latter in movement. Democritus

  misrepresents the facts when he expresses the opinion that if the

  interspace were empty one could distinctly see an ant on the vault

  of the sky; that is an impossibility. Seeing is due to an affection or

  change of what has the perceptive faculty, and it cannot be affected

  by the seen colour itself; it remains that it must be affected by what

  comes between. Hence it is indispensable that there be something in

  between-if there were nothing, so far from seeing with greater

  distinctness, we should see nothing at all.

  We have now explained the cause why colour cannot be seen

  otherwise than in light. Fire on the other hand is seen both in

  darkness and in light; this double possibility follows necessarily

  from our theory, for it is just fire that makes what is potentially

  transparent actually transparent.

  The same account holds also of sound and smell; if the object of

  either of these senses is in immediate contact with the organ no

  sensation is produced. In both cases the object sets in movement

  only what lies between, and this in turn sets the organ in movement:

  if what sounds or smells is brought into immediate contact with the

  organ, no sensation will be produced. The same, in spite of all

  appearances, applies also to touch and taste; why there is this

  apparent difference will be clear later. What comes between in the

  case of sounds is air; the corresponding medium in the case of smell

  has no name. But, corresponding to what is transparent in the case

  of colour, there is a quality found both in air and water, which

  serves as a medium for what has smell-I say 'in water' because animals

  that live in water as well as those that live on land seem to

  possess the sense of smell, and 'in air' because man and all other

  land animals that breathe, perceive smells only when they breathe

  air in. The explanation of this too will be given later.

  8

  Now let us, to begin with, make certain distinctions about sound and

  hearing.

  Sound may mean either of two things (a) actual, and (b) potential,

  sound. There are certain things which, as we say, 'have no sound',

  e.g. sponges or wool, others which have, e.g. bronze and in general

  all things which are smooth and solid-the latter are said to have a

  sound because they can make a sound, i.e. can generate actual sound

  between themselves and the organ of hearing.

  Actual sound requires for its occurrence (i, ii) two such bodies and

  (iii) a space between them; for it is generated by an impact. Hence it

  is impossible for one body only to generate a sound-there must be a

  body impinging and a body impinged upon; what sounds does so by

  striking against something else, and this is impossible without a

  movement from place to place.

  As we have said, not all bodies can by impact on one another produce

  sound; impact on wool makes no sound, while the impact on bronze or

  any body which is smooth and hollow does. Bronze gives out a sound

  when struck because it is smooth; bodies which are hollow owing to

  reflection repeat the original impact over and over again, the body

  originally set in movement being unable to escape from the concavity.

  Further, we must remark that sound is heard both in air and in

  water, though less distinctly in the latter. Yet neither air nor water

  is the principal cause of sound. What is required for the production

  of sound is an impact of two solids against one another and against

  the air. The latter condition is satisfied when the air impinged

  upon does not retreat before the blow, i.e. is not dissipated by it.

  That is why it must be struck with a sudden sharp blow, if it is

  to sound-the movement of the whip must outrun the dispersion of the

  air, just as one might get in a stroke at a heap or whirl of sand as

  it was traveling rapidly past.

  An echo occurs, when, a mass of air having been unified, bounded,

  and prevented from dissipation by the containing walls of a vessel,

  the air originally struck by the impinging body and set in movement by

  it rebounds from this mass of air like a ball from a wall. It is

  probable that in all generation of sound echo takes place, though it

  is frequently only indistinctly heard. What happens here must be

  analogous to what happens in the case of light; light is always

  reflected-otherwise it would not be diffused and outside what was

  directly illuminated by the sun there would be blank darkness; but

  this reflected light is not always strong enough, as it is when it

  is reflected from water, bronze, and other smooth bodies, to cast a

  shadow, which is the distinguishing mark by which we recognize light.

  It is rightly said that an empty space plays the chief part in the

  production of hearing, for what people mean by 'the vacuum' is the

  air, which is what causes hearing, when that air is set in movement as

  one continuous mass; but owing to its friability it emits no sound,

  being dissipated by impinging upon any surface which is not smooth.

  When the surface on which it impinges is quite smooth, what is

  produced by the original impact is a united mass, a result due to

  the smoothness of the surface with which the air is in contact at

  the other end.

  What has the power of producing sound is what has the power of

  setting in movement a single mass of air which is continuous from

  the impinging body up to the organ of hearing. The organ of hearing is

  physically united with air, and because it is in air, the air inside

  is moved concurrently with the air outside. Hence animals do not

  hear with all parts of their bodies, nor do all parts admit of the

  entrance of air; for even the part which can be moved and can sound

  has not air everywhere in it. Air in itself is, owing to its

  friability, quite soundless; only when its dissipation is prevented is

  its movement sound. The air in the ear is built into a chamber just to

  prevent this dissipating movement, in order that the animal may

  accurately apprehend all varieties of the movements of the air

  outside. That is why we hear also in water, viz. because the water

  cannot get into the air chamber or even, owing to the spirals, into

  the outer ear. If this does happen, hearing ceases, as it also does if

  the tympanic membrane is damaged, just as sight ceases if the membrane

  covering the pupil is damaged. It is also a test of deafness whether

  the ear does or does not reverberate like a horn; the air inside the

  ear has always a movement of its own, but the sound we hear is

  always the sounding of something else, not of the organ itself. That

  is why we say that we hear with what is empty and echoes, viz. because

  what we hear with is a chamber which contains a bounded mass of air.

  Which is it that 'sounds', the striking body or the struck? Is not

  the answer 'it is both, but each in a different way'? Sound is a

  movement of what can rebound
from a smooth surface when struck against

  it. As we have explained' not everything sounds when it strikes or

  is struck, e.g. if one needle is struck against another, neither emits

  any sound. In order, therefore, that sound may be generated, what is

  struck must be smooth, to enable the air to rebound and be shaken

  off from it in one piece.

  The distinctions between different sounding bodies show themselves

  only in actual sound; as without the help of light colours remain

  invisible, so without the help of actual sound the distinctions

  between acute and grave sounds remain inaudible. Acute and grave are

  here metaphors, transferred from their proper sphere, viz. that of

  touch, where they mean respectively (a) what moves the sense much in a

  short time, (b) what moves the sense little in a long time. Not that

  what is sharp really moves fast, and what is grave, slowly, but that

  the difference in the qualities of the one and the other movement is

  due to their respective speeds. There seems to be a sort of

  parallelism between what is acute or grave to hearing and what is

  sharp or blunt to touch; what is sharp as it were stabs, while what is

  blunt pushes, the one producing its effect in a short, the other in

  a long time, so that the one is quick, the other slow.

  Let the foregoing suffice as an analysis of sound. Voice is a kind

  of sound characteristic of what has soul in it; nothing that is

  without soul utters voice, it being only by a metaphor that we speak

  of the voice of the flute or the lyre or generally of what (being

  without soul) possesses the power of producing a succession of notes

  which differ in length and pitch and timbre. The metaphor is based

  on the fact that all these differences are found also in voice. Many

  animals are voiceless, e.g. all non-sanuineous animals and among

  sanguineous animals fish. This is just what we should expect, since

  voice is a certain movement of air. The fish, like those in the

  Achelous, which are said to have voice, really make the sounds with