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of the several senses。
12
The following results applying to any and every sense may now be formulated。 （A） By a 'sense' is meant what has the power of receiving into itself the sensible forms of things without the matter。 This must be conceived of as taking place in the way in which a piece of wax takes on the impress of a signet…ring without the iron or gold； we say that what produces the impression is a signet of bronze or gold； but its particular metallic constitution makes no difference： in a similar way the sense is affected by what is coloured or flavoured or sounding； but it is indifferent what in each case the substance is； what alone matters is what quality it has； i。e。 in what ratio its constituents are combined。 （B） By 'an organ of sense' is meant that in which ultimately such a power is seated。 The sense and its organ are the same in fact； but their essence is not the same。 What perceives is； of course； a spatial magnitude； but we must not admit that either the having the power to perceive or the sense itself is a magnitude； what they are is a certain ratio or power in a magnitude。 This enables us to explain why objects of sense which possess one of two opposite sensible qualities in a degree largely in excess of the other opposite destroy the organs of sense； if the movement set up by an object is too strong for the organ； the equipoise of contrary qualities in the organ； which just is its sensory power； is disturbed； it is precisely as concord and tone are destroyed by too violently twanging the strings of a lyre。 This explains also why plants cannot perceive。 in spite of their having a portion of soul in them and obviously being affected by tangible objects themselves； for undoubtedly their temperature can be lowered or raised。 The explanation is that they have no mean of contrary qualities； and so no principle in them capable of taking on the forms of sensible objects without their matter； in the case of plants the affection is an affection by form…and…matter together。 The problem might be raised： Can what cannot smell be said to be affected by smells or what cannot see by colours； and so on？ It might be said that a smell is just what can be smelt； and if it produces any effect it can only be so as to make something smell it； and it might be argued that what cannot smell cannot be affected by smells and further that what can smell can be affected by it only in so far as it has in it the power to smell （similarly with the proper objects of all the other senses）。 Indeed that this is so is made quite evident as follows。 Light or darkness； sounds and smells leave bodies quite unaffected； what does affect bodies is not these but the bodies which are their vehicles； e。g。 what splits the trunk of a tree is not the sound of the thunder but the air which accompanies thunder。 Yes； but； it may be objected； bodies are affected by what is tangible and by flavours。 If not； by what are things that are without soul affected； i。e。 altered in quality？ Must we not； then； admit that the objects of the other senses also may affect them？ Is not the true account this； that all bodies are capable of being affected by smells and sounds； but that some on being acted upon； having no boundaries of their own； disintegrate； as in the instance of air； which does become odorous； showing that some effect is produced on it by what is odorous？ But smelling is more than such an affection by what is odorous…what more？ Is not the answer that； while the air owing to the momentary duration of the action upon it of what is odorous does itself become perceptible to the sense of smell； smelling is an observing of the result produced？
Book III 1
THAT there is no sixth sense in addition to the five enumerated…sight； hearing； smell； taste； touch…may be established by the following considerations： If we have actually sensation of everything of which touch can give us sensation （for all the qualities of the tangible qua tangible are perceived by us through touch）； and if absence of a sense necessarily involves absence of a sense…organ； and if （1） all objects that we perceive by immediate contact with them are perceptible by touch； which sense we actually possess； and （2） all objects that we perceive through media； i。e。 without immediate contact； are perceptible by or through the simple elements； e。g。 air and water （and this is so arranged that （a） if more than one kind of sensible object is perceivable through a single medium； the possessor of a sense…organ homogeneous with that medium has the power of perceiving both kinds of objects； for example； if the sense…organ is made of air； and air is a medium both for sound and for colour； and that （b） if more than one medium can transmit the same kind of sensible objects； as e。g。 water as well as air can transmit colour； both being transparent； then the possessor of either alone will be able to perceive the kind of objects transmissible through both）； and if of the simple elements two only； air and water； go to form sense…organs （for the pupil is made of water； the organ of hearing is made of air； and the organ of smell of one or other of these two； while fire is found either in none or in all…warmth being an essential condition of all sensibility…and earth either in none or； if anywhere； specially mingled with the components of the organ of touch； wherefore it would remain that there can be no sense…organ formed of anything except water and air）； and if these sense…organs are actually found in certain animals；…then all the possible senses are possessed by those animals that are not imperfect or mutilated （for even the mole is observed to have eyes beneath its skin）； so that； if there is no fifth element and no property other than those which belong to the four elements of our world； no sense can be wanting to such animals。 Further； there cannot be a special sense…organ for the common sensibles either； i。e。 the objects which we perceive incidentally through this or that special sense； e。g。 movement； rest； figure； magnitude； number； unity； for all these we perceive by movement； e。g。 magnitude by movement； and therefore also figure （for figure is a species of magnitude）； what is at rest by the absence of movement： number is perceived by the negation of continuity； and by the special sensibles； for each sense perceives one class of sensible objects。 So that it is clearly impossible that there should be a special sense for any one of the common sensibles； e。g。 movement； for； if that were so； our perception of it would be exactly parallel to our present perception of what is sweet by vision。 That is so because we have a sense for each of the two qualities； in virtue of which when they happen to meet in one sensible object we are aware of both contemporaneously。 If it were not like this our perception of the common qualities would always be incidental； i。e。 as is the perception of Cleon's son； where we perceive him not as Cleon's son but as white； and the white thing which we really perceive happens to be Cleon's son。 But in the case of the common sensibles there is already in us a general sensibility which enables us to perceive them directly； there is therefore no special sense required for their perception： if there were； our perception of them would have been exactly like what has been above described。 The senses perceive each other's special objects incidentally； not because the percipient sense is this or that special sense； but because all form a unity： this incidental perception takes place whenever sense is directed at one and the same moment to two disparate qualities in one and the same object； e。g。 to the bitterness and the yellowness of bile； the assertion of the identity of both cannot be the act of either of the senses； hence the illusion of sense； e。g。 the belief that if a thing is yellow it is bile。 It might be asked why we have more senses than one。 Is it to prevent a failure to apprehend the common sensibles； e。g。 movement； magnitude； and number； which go along with the special sensibles？ Had we no sense but sight； and that sense no object but white； they would have tended to escape our notice and everything would have merged for us into an indistinguishable identity because of the concomitance of colour and magnitude。 As it is； the fact that the common sensibles are given in the objects of more than one sense reveals their distinction from each and all of the special sensibles。
2
Since it is through sense that we are aware that we are seeing or hearing； it must be either by sight that we are aware of seeing； or by some sense other than sight。 But the sense that gives us this new sensation must perceive both sight and its object； viz。 colour： so that either （1） there will be two senses both percipient of the same sensible object； or （2） the sense must be percipient of itself。 Further； even if the sense which perceives sight were different from sight； we must either fall into an infinite regress； or we must somewhere assume a sense which is aware of itself。 If so； we ought to do this in the first case。 This presents a difficulty： if to perceive by