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exercise of sense…perception takes place； since sense…perception； as
realized in actual perceiving； is a mode of qualitative change。 This
explains why the affection continues in the sensory organs； both in
their deeper and in their more superficial parts； not merely while
they are actually engaged in perceiving； but even after they have
ceased to do so。 That they do this； indeed； is obvious in cases
where we continue for some time engaged in a particular form of
perception； for then； when we shift the scene of our perceptive
activity； the previous affection remains； for instance； when we have
turned our gaze from sunlight into darkness。 For the result of this is
that one sees nothing； owing to the excited by the light still
subsisting in our eyes。 Also； when we have looked steadily for a
long while at one colour； e。g。 at white or green； that to which we
next transfer our gaze appears to be of the same colour。 Again if；
after having looked at the sun or some other brilliant object； we
close the eyes； then； if we watch carefully； it appears in a right
line with the direction of vision （whatever this may be）； at first
in its own colour； then it changes to crimson； next to purple； until
it becomes black and disappears。 And also when persons turn away
from looking at objects in motion； e。g。 rivers； and especially those
which flow very rapidly； they find that the visual stimulations
still present themselves； for the things really at rest are then
seen moving： persons become very deaf after hearing loud noises； and
after smelling very strong odours their power of smelling is impaired；
and similarly in other cases。 These phenomena manifestly take place in
the way above described。
That the sensory organs are acutely sensitive to even a slight
qualitative difference ＇in their objects＇ is shown by what happens
in the case of mirrors； a subject to which； even taking it
independently； one might devote close consideration and inquiry。 At
the same time it becomes plain from them that as the eye ＇in seeing＇
is affected ＇by the object seen＇； so also it produces a certain effect
upon it。 If a woman chances during her menstrual period to look into a
highly polished mirror； the surface of it will grow cloudy with a
blood…coloured haze。 It is very hard to remove this stain from a new
mirror； but easier to remove from an older mirror。 As we have said
before； the cause of this lies in the fact that in the act of sight
there occurs not only a passion in the sense organ acted on by the
polished surface； but the organ； as an agent； also produces an action；
as is proper to a brilliant object。 For sight is the property of an
organ possessing brilliance and colour。 The eyes； therefore； have
their proper action as have other parts of the body。 Because it is
natural to the eye to be filled with blood…vessels； a woman's eyes；
during the period of menstrual flux and inflammation； will undergo a
change； although her husband will not note this since his seed is of
the same nature as that of his wife。 The surrounding atmosphere；
through which operates the action of sight； and which surrounds the
mirror also； will undergo a change of the same sort that occurred
shortly before in the woman's eyes； and hence the surface of the
mirror is likewise affected。 And as in the case of a garment； the
cleaner it is the more quickly it is soiled； so the same holds true in
the case of the mirror。 For anything that is clean will show quite
clearly a stain that it chances to receive； and the cleanest object
shows up even the slightest stain。 A bronze mirror； because of its
shininess； is especially sensitive to any sort of contact （the
movement of the surrounding air acts upon it like a rubbing or
pressing or wiping）； on that account； therefore； what is clean will
show up clearly the slightest touch on its surface。 It is hard to
cleanse smudges off new mirrors because the stain penetrates deeply
and is suffused to all parts； it penetrates deeply because the
mirror is not a dense medium； and is suffused widely because of the
smoothness of the object。 On the other hand； in the case of old
mirrors； stains do not remain because they do not penetrate deeply；
but only smudge the surface。
From this therefore it is plain that stimulatory motion is set up
even by slight differences； and that sense…perception is quick to
respond to it； and further that the organ which perceives colour is
not only affected by its object； but also reacts upon it。 Further
evidence to the same point is afforded by what takes place in wines；
and in the manufacture of unguents。 For both oil； when prepared； and
wine become rapidly infected by the odours of the things near them；
they not only acquire the odours of the things thrown into or mixed
with them； but also those of the things which are placed； or which
grow； near the vessels containing them。
In order to answer our original question； let us now； therefore；
assume one proposition； which is clear from what precedes； viz。 that
even when the external object of perception has departed； the
impressions it has made persist； and are themselves objects of
perception： and ＇let us assume＇； besides； that we are easily
deceived respecting the operations of sense…perception when we are
excited by emotions； and different persons according to their
different emotions； for example； the coward when excited by fear；
the amorous person by amorous desire； so that； with but little
resemblance to go upon； the former thinks he sees his foes
approaching； the latter； that he sees the object of his desire； and
the more deeply one is under the influence of the emotion； the less
similarity is required to give rise to these illusory impressions。
Thus too； both in fits of anger； and also in all states of appetite；
all men become easily deceived； and more so the more their emotions
are excited。 This is the reason too why persons in the delirium of
fever sometimes think they see animals on their chamber walls； an
illusion arising from the faint resemblance to animals of the markings
thereon when put together in patterns； and this sometimes
corresponds with the emotional states of the sufferers； in such a
way that； if the latter be not very ill； they know well enough that it
is an illusion； but if the illness is more severe they actually move
according to the appearances。 The cause of these occurrences is that
the faculty in virtue of which the controlling sense judges is not
identical with that in virtue of which presentations come before the
mind。 A proof of this is； that the sun presents itself as only a
foot in diameter； though often something else gainsays the
presentation。 Again； when the fingers are crossed； the one object
＇placed between them＇ is felt ＇by the touch＇ as two； bu