by Aristotle
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
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