Among the five senses, Aristotle distinguishes two as operating by direct contact between subject and object (touch, taste); three as operating through an external intervening medium (vision, smell, taste). He begins with Vision, which he regards as possessing most completely the nature and characteristics of a sense.76 The direct and proper object of vision is colour. Now colour operates upon the eye not immediately (for, if the coloured object be placed in contact with the eye, there will be no vision), but by causing movements or perturbations in the external intervening medium, air or water, which affect the sense through an appropriate agency of their own.77 This agency is, according to Aristotle, the Diaphanous or Transparent. When actual or in energy, the transparent is called light; when potential or in capacity only, it is called darkness. The eye is of watery structure, apt for receiving these impressions.78 It is the presence either of fire, or of something analogous to the celestial body, that calls forth the diaphanous from the state of potentiality into that of actuality or light; in which latter condition it is stimulated by colour. The diaphanous, whether as light or as darkness, is a peculiar nature or accompaniment, not substantive in itself, but inherent chiefly in the First or Celestial Body, yet also in air, water, glass, precious stones, and in all bodies to a greater or less degree.79 The diaphanous passes at once and simultaneously, in one place as well as in another, from potentiality to actuality — from darkness to light. Light does not take time to travel from one place to another, as sound and smell do.80 The diaphanous is not a body, nor effluvium from a body, nor any one of the elements: it is of an adjective character — a certain agency or attribute pervading or belonging to bodies, along with their extension.81 Colour marks and defines the surface of the body quâ diaphanous, as figure defines it quâ extended. Colour makes the diaphanous itself visible, and its own varieties visible through the diaphanous. Air and water are transparent throughout, though with an ill-defined superficial colour. White and black, as colours in solid bodies, correspond to the condition of light or darkness in air. There are some luminous objects visible in the dark, as fire, fungous matter, eyes, and scales of fish, &c., though they have no appropriate colour.82 There are seven species or varieties of colours, but all of them proceed from white and black, blended in different proportions, or seen one through another; white and black are the two extremes, the other varieties being intermediate between them.

The same necessity for an intervening medium external to the subject, as in the case of vision, prevails also in the senses of hearing and smell. If the audible or odorous object be placed in contact with its organ of sense, there will be no hearing or smell. Whenever we hear or smell any object, there must be interposed between us and the object a suitable medium that shall be affected first; while the organ of sense will be affected secondarily through that medium. Air is the medium in regard to sound, both air and water in regard to smell; but there seems besides (analogous to the transparent in regard to vision) a special agency called the Trans-Sonant, which pervades air and enables it to transmit sound; and certainly another special agency called the Trans-Olfacient, which pervades both air and water, and enables them to transmit smell.83 (It seems thus that something like a luminiferous ether — extended, mobile, and permeating bodies, yet still incorporeal in itself — was an hypothesis as old as Aristotle; and one other ether besides, analogous in property and purpose — an odoriferous ether; perhaps a third or soniferous ether, but this is less distinctly specified by Aristotle.)

Sound, according to Aristotle, arises from the shock of two or more solid bodies communicated to the air. It implies local movement in one at least of those bodies. Many soft bodies are incapable of making sound; those best suited for it are such as metals, hard in structure, smooth in surface, hollow in shape. The blow must be smart and quick, otherwise the air slips away and dissipates itself before the sound can be communicated to it.84 Sound is communicated through the air to the organ of hearing; the air is one continuum (not composed of adjacent particles with interspaces), and a wave is propagated from it to the internal ear, which contains some air enclosed in the sinuous ducts within the membrane of the tympanum, congenitally attached to the organ itself, and endued with a certain animation.85 This internal air within the ear, excited by the motion propagated from the external ear, causes hearing. The ear is enabled to appreciate accurately the movements of the external air, because it has itself little or no movement within. We cannot hear with any other part of the body; because it is only in the ear that nature has given us this stock of internal air. If water gets into the ear, we cannot hear at all; because the wave generated in the air without, cannot propagate itself within. Nor can we hear, if the membrane of the ear be disordered; any more than we can see, when the membrane of the eye is disordered.86

Voice is a kind of sound peculiar to animated beings; yet not belonging to all of them, but only to those that inspire the air. Nature employs respiration for two purposes: the first, indispensable to animal life, — that of cooling and tempering the excessive heat of the heart and its adjacent parts; the second, not indispensable to life, yet most valuable to the higher faculties of man, — significant speech. The organ of respiration is the larynx; a man cannot speak either when inspiring or expiring, but only when retaining and using the breath within. The soul in those parts, when guided by some phantasm or thought, impels the air within against the walls of the trachea, and this shock causes vocal sounds.87

Aristotle seems to have been tolerably satisfied with the above explanation of sight and hearing; for, in approaching the sense of Smell with the olfacients, he begins by saying that it is less definable and explicable. Among the five senses, smell stands intermediate between the two (taste and touch) that operate by direct contact, and the other two (sight and hearing) that operate through an external medium. Man is below other animals in this sense; he discriminates little in smells except the pleasurable and the painful.88 His taste, though analogous in many points to smell, is far more accurate and discriminating, because taste is a variety of touch; and in respect to touch, man is the most discriminating of all animals. Hence his great superiority to them in practical wisdom. Indeed the marked difference of intelligence between one man and another, turns mainly upon the organ of touch: men of hard flesh (or skin) are by nature dull in intelligence, men of soft flesh are apt and clever.89 The classifying names of different smells are borrowed from the names of the analogous tastes to which they are analogous — sweet, bitter, tart, dry, sharp, smooth, &c.90 Smells take effect through air as well as through water; by means of a peculiar agency or accompaniment (mentioned above, called the Trans-Olfacient) pervading both one and the other. It is peculiar to man that he cannot smell except when inhaling air in the act of inspiration; any one may settle this for himself by making the trial.91 But fishes and other aquatic animals, which never inhale air, can smell in the water; and this proves that the trans-olfacient agency is operative to transmit odours not less in water than in air.92 We know that the sense of smell in these aquatic animals is the same as it is in man, because the same strong odours that are destructive to man are also destructive to them.93 Smell is the parallel, and in a certain sense the antithesis of taste; smell is of the dry, taste is of the moist: the olfactory matter is a juicy or sapid dryness, extracted or washed out from both air and water by the trans-olfacient agency, and acting on the sensory potentialities of the nostrils.94 This olfactory inhalation is warm as well as dry. Hence it is light, and rises easily to the brain, the moisture and coldness of which it contributes to temper; this is a very salutary process, for the brain is the wettest and coldest part of the body, requiring warm and dry influences as a corrective. It is with a view to this correction that Nature has placed the olfactory organ in such close proximity to the brain.95 There are two kinds of olfactory impressions. One of them is akin to the sense of taste — odour and savour going together — an affection (to a great degree) of the nutritive soul; so that the same odour is agreeable when we are hungry, disagreeable when our hunger is fully satisfied. This first kind of impression is common to men with other animals; but there is a second, peculiar to man, and disconnected from the sense of taste, viz., the scent of flowers, unguents, &c., which are agreeable or disagreeable constantly and per se.96 Nature has assigned this second kind of odours as a privilege to man, because his brain, being so large and moist, requires to be tempered by an additional stock of drying and warming olfactory influence.

Taste is a variety of touch, and belongs to the lower or nutritive soul, as a guide to the animal in seeking or avoiding different sorts of food. The object of taste is essentially liquid, often strained and extracted from dry food by warmth and moisture. The primary manifestation of this sensory phenomenon is the contrast of drinkable and undrinkable.97 The organ of taste, the tongue, is a mean between dryness and moisture; when either of these is in excess, the organ is disordered. Among the varieties of taste, there are two fundamental contraries (as in colour, sound, and the objects of the other senses except touch) from which the other contrasts are derived. These fundamentals in taste are sweet and bitter; corresponding to white and black, acute and grave, in colours and sounds. The sense of taste is potentially sweet or bitter; the gustable object is what makes it sweet or bitter in actuality.98

The sense of touch, in which man surpasses all other animals, differs from the other senses by not having any two fundamental contraries giving origin to the rest, but by having various contraries alike fundamental. It is thus hardly one sense, but an aggregate of several senses. It appreciates the elementary differences of body quâ body — hot, cold, dry, moist, hard, soft, &c. It is a mean between each of these two extremes; being potentially either one of them, and capable of being made to assimilate itself actually to either.99 In this sense, the tangible object operates when in contact with the skin; and, as has been already said, much of the superiority of man depends upon his superior fineness and delicacy of skin.100 Still Aristotle remarks that the true organ of touch is not the skin or flesh, but something interior to the flesh. This last serves only as a peculiar medium. The fact that the sensation arises when the object touches our skin, does not prove that the skin is the true organ; for, if there existed a thin exterior membrane surrounding our bodies, we should still feel the same sensation. Moreover, the body is not in real contact with our skin, though it appears to be so; there is a thin film of air between the two, though we do not perceive it; just as, when we touch an object under water, there is a film of water interposed between, as is seen by the wetness of the finger.101 The skin is, therefore, not the true organ of touch, but a medium between the object and the organ; and this sense does in reality agree with the other senses in having a certain medium interposed between object and organ. But there is this difference: in touch the medium is close to and a part of ourselves; in sight and hearing it is exterior to ourselves, and may extend to some distance. In sight and hearing the object does not affect us directly; it affects the external medium, which again affects us. But in touch the object affects, at the same time and by the same influence, both the medium and the interior organ; like a spear that, with the same thrust, pierces the warrior’s shield and wounds the warrior himself.102 Apparently, therefore, the true organ of touch is something interior, and skin and flesh is an interposed medium.103 But what this interior organ is, Aristotle does not more particularly declare. He merely states it to be in close and intimate communication with the great central focus and principle of all sensation — the heart;104 more closely connected with the heart (he appears to think) than any of the other organs of sense, though all of them are so connected more or less closely.

Having gone through the five senses seriatim, Aristotle offers various reasons to prove that there neither are, nor can be, more than five; and then discusses some complicated phenomena of sense. We perceive that we see or hear;105 do we perceive this by sight or by hearing? and if not, by what other faculty?106 Aristotle replies by saying that the act of sense is one and the same, but that it may be looked at in two different points of view. We see a coloured object; we hear a sound: in each case the act of sense is one; the energy or actuality of the visum, and videns, of the sonans and audiens, is implicated and indivisible. But the potentiality of the one is quite distinct from the potentiality of the other, and may be considered as well as named apart.107 When we say: I perceive that I see — we look at the same act of vision from the side of the videns; the visum being put out of sight as the unnoticed correlate. This is a mental fact distinct from, though following upon, the act of vision itself. Aristotle refers it rather to that general sentient soul or faculty, of which the five senses are partial and separate manifestations, than to the sense of vision itself.108 He thus considers what would now be termed consciousness of a sensation, as being merely the subjective view of the sensation, distinguished by abstraction from the objective.

It is the same general sentient faculty, though diversified and logically distinguishable in its manifestations, that enables us to conceive many sensations as combined into one; and to compare or discriminate sensations belonging to different senses.109

White and sweet are perceived by two distinct senses, and at two distinct moments of time; but they must be compared and discriminated by one and the same sentient or cogitant act, and at one moment of time.110 This mental act, though in itself indivisible, has yet two aspects, and is thus in a certain sense divisible; just as a point taken in the middle of a line, while indivisible in itself, may be looked upon as the closing terminus of one-half of the line, and as the commencing terminus of the other half. The comparison of two different sensations or thoughts is thus one and the same mental fact, with two distinguishable aspects.111