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Sensation is that faculty of the soul whereby material objects can be
apprehended or discriminated. And the sensoria are the organs or
members through which sensations are conveyed. And the objects of
sense are the things that come within the province of sensation. And
lastly, the subject of sense is the living animal which possesses the
faculty of sensation. Now there are five senses, and likewise five
organs of sense.
The first sense is sight: and the sensoria or organs of sight are the
nerves of the brain and the eyes. Now sight is primarily perception of
colour, but along with the colour it discriminates the body that has
colour, and its size and form, and locality, and the intervening
space and the number: also whether it is in motion or at rest, rough
or smooth, even or uneven, sharp or blunt, and finally whether its
composition is watery or earthy, that is, wet or dry.
The second sense is hearing, whereby voices and sounds are perceived.
And it distinguishes these as sharp or deep, or smooth or loud. Its
organs are the soft nerves of the brain, and the structure of the
ears. Further, man and the ape are the only animals that do not move
their ears.
The third sense is smell, which is caused by the nostrils transmitting
the vapours to the brain: and it is bounded by the extreme limits of
the anterior ventricle of the brain. It is the faculty by which
vapours are perceived and apprehended. Now, the most generic
distinction between vapours is whether they have a good or an evil
odour, or form an intermediate class with neither a good nor an evil
odour. A good odour is produced by the thorough digestion in the body
of the humours. When they are only moderately digested the
intermediate class is formed, and when the digestion is very imperfect
or utterly wanting, an evil odour results.
The fourth sense is taste: it is the faculty whereby the humours are
apprehended or perceived, and its organs of sense are the tongue, and
more especially the lips, and the palate (which the Greeks call
ouraniskou), and in these are nerves that come
from the brain and are spread out, and convey to the dominant part of
the soul the perception or sensation they have encountered. The
so-called gustatory qualities of the humours are these:--sweetness,
pungency, bitterness, astringency, acerbity, sourness, saltness,
fattiness, stickiness; for taste is capable of discriminating all
these. But water has none of these qualities, and is therefore devoid
of taste. Moreover, astringency is only a more intense and
exaggerated form of acerbity.
The fifth sense is touch, which is common to all living things. Its
organs are nerves which come from the brain and ramify all through the
body. Hence the body as a whole, including even the other organs of
sense, possesses the sense of touch. Within its scope come heat and
cold, softness and hardness, viscosity and brittleness, heaviness and
lightness: for it is by touch alone that these qualities are
discriminated. On the other hand, roughness and smoothness, dryness
and wetness, thickness and thinness, up and down, place and size,
whenever that is such as to be embraced in a single application of the
sense of touch, are all common to touch and sight, as well as
denseness and rareness, that is porosity, and rotundity if it is
small, and some other shapes. In like manner also by the aid of
memory and thought perception of the nearness of a body is possible,
and similarly perception of number up to two or three, and such small
and easily reckoned figures. But it is by sight rather than touch that
these things are perceived.
The Creator, it is to be noted, fashioned all the other organs of
sense in pairs, so that if one were destroyed, the other might fill
its place. For there are two eyes, two ears, two orifices of the
nose, and two tongues, which in some animals, such as snakes, are
separate, but in others, like man, are united. But touch is spread
over the whole body with the exception of bones, nerves, nails,
horns, hairs, ligaments, and other such structures.
Further, it is to be observed that sight is possible only in straight
lines, whereas smell and hearing are not limited to straight lines
only, but act in all directions. Touch, again, and taste act
neither in straight lines, nor in every direction, but only when each
comes near to the sensible objects that are proper to it.
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