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animals only while respiring （for that this is the fact is manifest in
all the animals which do respire）； whereas none of those just
mentioned respires； and yet they have the sense of smell… unless；
indeed； they have some other sense not included in the ordinary
five。 This supposition is； however； impossible。 For any sense which
perceives odour is a sense of smell； and this they do perceive； though
probably not in the same way as creatures which respire； but when
the latter are respiring the current of breath removes something
that is laid like a lid upon the organ proper （which explains why they
do not perceive odours when not respiring）； while in creatures which
do not respire this is always off： just as some animals have eyelids
on their eyes； and when these are not raised they cannot see；
whereas hard…eyed animals have no lids； and consequently do not
need； besides eyes； an agency to raise the lids； but see straightway
＇without intermission＇ from the actual moment at which it is first
possible for them to do so ＇i。e。 from the moment when an object
first comes within their field of vision＇。
Consistently with what has been said above； not one of the lower
animals shows repugnance to the odour of things which are
essentially ill…smelling； unless one of the latter is positively
pernicious。 They are destroyed； however； by these things； just as
human beings are； i。e。 as human beings get headaches from； and are
often asphyxiated by； the fumes of charcoal； so the lower animals
perish from the strong fumes of brimstone and bituminous substances；
and it is owing to experience of such effects that they shun these。
For the disagreeable odour in itself they care nothing whatever
（though the odours of many plants are essentially disagreeable）；
unless； indeed； it has some effect upon the taste of their food。
The senses making up an odd number； and an odd number having
always a middle unit； the sense of smell occupies in itself as it were
a middle position between the tactual senses； i。e。 Touch and Taste；
and those which perceive through a medium； i。e。 Sight and Hearing。
Hence the object of smell； too； is an affection of nutrient substances
（which fall within the class of Tangibles）； and is also an affection
of the audible and the visible； whence it is that creatures have the
sense of smell both in air and water。 Accordingly； the object of smell
is something common to both of these provinces； i。e。 it appertains
both to the tangible on the one hand； and on the other to the
audible and translucent。 Hence the propriety of the figure by which it
has been described by us as an immersion or washing of dryness in
the Moist and Fluid。 Such then must be our account of the sense in
which one is or is not entitled to speak of the odorous as having
species。
The theory held by certain of the Pythagoreans； that some animals
are nourished by odours alone； is unsound。 For； in the first place； we
see that food must be composite； since the bodies nourished by it
are not simple。 This explains why waste matter is secreted from
food； either within the organisms； or； as in plants； outside them。 But
since even water by itself alone； that is； when unmixed； will not
suffice for food… for anything which is to form a consistency must be
corporeal…； it is still much less conceivable that air should be so
corporealized ＇and thus fitted to be food＇。 But； besides this； we
see that all animals have a receptacle for food； from which； when it
has entered； the body absorbs it。 Now； the organ which perceives odour
is in the head； and odour enters with the inhalation of the breath； so
that it goes to the respiratory region。 It is plain； therefore； that
odour； qua odour； does not contribute to nutrition； that； however；
it is serviceable to health is equally plain； as well by immediate
perception as from the arguments above employed； so that odour is in
relation to general health what savour is in the province of nutrition
and in relation to the bodies nourished。
This then must conclude our discussion of the several organs of
sense…perception。
6
One might ask： if every body is infinitely divisible； are its
sensible qualities… Colour； Savour； Odour； Sound； Weight； Cold or
Heat； ＇Heaviness or＇ Lightness； Hardness or Softness…also infinitely
divisible？ Or； is this impossible？
＇One might well ask this question＇； because each of them is
productive of sense…perception； since； in fact； all derive their
name ＇of 'sensible qualities'＇ from the very circumstance of their
being able to stimulate this。 Hence； ＇if this is so＇ both our
perception of them should likewise be divisible to infinity； and every
part of a body ＇however small＇ should be a perceptible magnitude。
For it is impossible； e。g。 to see a thing which is white but not of
a certain magnitude。
Since if it were not so； ＇if its sensible qualities were not
divisible； pari passu with body＇； we might conceive a body existing
but having no colour； or weight； or any such quality； accordingly
not perceptible at all。 For these qualities are the objects of
sense…perception。 On this supposition； every perceptible object should
be regarded as composed not of perceptible ＇but of imperceptible＇
parts。 Yet it must ＇be really composed of perceptible parts＇； since
assuredly it does not consist of mathematical ＇and therefore purely
abstract and non…sensible＇ quantities。 Again； by what faculty should
we discern and cognize these ＇hypothetical real things without
sensible qualities＇？ Is it by Reason？ But they are not objects of
Reason； nor does reason apprehend objects in space； except when it
acts in conjunction with sense…perception。 At the same time； if this
be the case ＇that there are magnitudes； physically real； but without
sensible quality＇； it seems to tell in favour of the atomistic
hypothesis； for thus； indeed； ＇by accepting this hypothesis＇； the
question ＇with which this chapter begins＇ might be solved
＇negatively＇。 But it is impossible ＇to accept this hypothesis＇。 Our
views on the subject of atoms are to be found in our treatise on
Movement。
The solution of these questions will bring with it also the answer
to the question why the species of Colour； Taste； Sound； and other
sensible qualities are limited。 For in all classes of things lying
between extremes the intermediates must be limited。 But contraries are
extremes； and every object of sense…perception involves contrariety：
e。g。 in Colour； White x Black； in Savour； Sweet x Bitter； and in all
the other sensibles also the contraries are extremes。 Now； that
which is continuous is divisible into an infinite number of unequal
parts； but into a finite number of equal parts； while that which is
not per se continuous is divisible into species which are finite in
number。 Since then； the several sensible qualities of things are to be
reckoned as species； while continuity always subsists in these； we
must take account of the difference between the Potential and the
Actual。 It is owing to this difference that we do not ＇actually＇ see
its ten…thousandth part in a grain of millet； although sight has
embraced the whole grain within its scope； and it is owing to this；
too； that the sound contained in a quarter…tone escapes notice； and
yet one hears the whole strain； inasmuch as it is a continuum； but the
interval between the extreme sounds ＇that bound the quarter…tone＇
escapes the ear ＇being only potentially audible； not actually＇。 So； in
the case of other objects of sense； extremely small constituents are
unnoticed； because they are only potentially not actually ＇perceptible
e。g。＇ visible； unless when they have been parted from the wholes。 So
the footlength too exists