Subsection 6. The sensitive soul.

Next in order comes the sensitive faculty, which exceeds the other in dignity as much as an animal ranks above a plant, since it includes those vegetative powers. It is defined as the actuality of an organized body by which that body lives, senses, desires, judges, breathes and moves. Its object in general is a perceptible or “passible” quality, since the sense is affected by it. Its common organ is the brain, from which the operations of sensation principally arise.

This sensitive soul is divided into two powers: perception and movement. Through the perceptive power we apprehend the sensory forms of things present or absent, and retain them as wax retains the impression of a seal. Through the moving power, the body is carried externally from one place to another, or moved internally by the spirits and the pulse.

The perceptive faculty is subdivided into two parts, internal and external. The external senses are the five of touch, hearing, sight, smell and taste. To these you may add Scaliger’s sixth sense of tickling, if you please, or speech, which Lullius considers the sixth external sense. There are three internal senses: common sense, imagination and memory.

The five external senses have as their objects only external things that are present: the eye sees no colour unless it is within reach, nor does the ear hear a sound. Three of these senses—hearing, sight and smell—are useful; two—touch and taste—are necessary, and without them we cannot live.

Moreover, the sensitive power is active or passive. It is active in sight when the eye sees a colour; passive when its object harms it, as the sun’s rays harm the eye. Hence the axiom: an excessively powerful visible object destroys the sense. Or the object may be unpleasant, as a disagreeable sound is to the ear, a stench to the nose, and so on.

Of these five senses, sight is considered the most precious and the best because of its object: it sees the whole body at once. Through it we learn and distinguish all things; it is a sense of outstanding usefulness. Sight requires three things: the object, the organ and the medium. Its object in general is the visible, or whatever can be seen, such as colours and all shining bodies.

The medium is the illumination of the air by light, commonly called the transparent medium, for we cannot see in the dark. The organ is the eye, especially its pupil, which conveys sight to the common sense through the two optic nerves, meeting in one. A suitable distance is needed between organ and object: the object must be neither too near nor too far away.

Philosophers discuss many excellent questions concerning this sense: whether sight is produced by taking in visible forms or sending them out, for instance, a matter debated by Plato, Plutarch, Macrobius, Lactantius and others. It is also the subject of optics, on which the Arab Alhazen, Witelo, Roger Bacon, Giambattista della Porta, Guidobaldo and Aguilonius, among others, have written whole volumes.

Hearing is an excellent external sense through which we learn and gain knowledge. Its object is sound, or what is heard; its medium, air; its organ, the ear. Sound, which is a collision of air, requires three things: a body to strike, such as a musician’s hand; a body to be struck, which must be solid and offer resistance, such as a bell or lute string, not wool or sponge; and the medium, air, which is internal or external.

External air, struck or driven together by a solid body, strikes the air next to it in succession until it reaches the natural air within. This internal air is contained, as a finely sensitive instrument, in a small membrane shaped like a drumhead. Struck by certain little instruments like drumsticks, it conveys sound through a pair of nerves assigned to that purpose to the common sense, the judge of sounds. Sounds have great variety and offer much delight; for knowledge of them, consult Boethius and other writers on music.

Smell is an external sense that perceives through the nostrils drawing in air. Of all the senses it is the weakest in human beings. Its organ is the nose, or two small hollow pieces of flesh a little above it; its medium is air for humans, as water is for fish. Its object is odour, arising from a compound body as it breaks down. Whether this is a quality, fume, vapour or exhalation, and how these differ or are produced, I will not now discuss.

This sense is an instrument of health, as sight and hearing, says Aulus Gellius, are instruments of learning. It serves health by avoiding bad smells and choosing good ones, which often alter and affect the body as much as diet itself.

Taste is a necessary sense that perceives all flavours through the tongue and palate, by means of thin saliva or a watery fluid. Its organ is the tongue with its nerves of taste; its medium, a watery fluid; its object, taste or flavour, a quality in the fluid arising from the mixture of things tasted. Some distinguish eight kinds of flavour: bitter, sweet, sharp, salty, and so on. Sick people, as in a fever, cannot distinguish all these because their organs are impaired.

Touch is the last and least noble of the senses, yet as necessary and pleasurable as the other. This sense is finely developed in humans and perceives every tangible quality through its nerves, distributed throughout the body. Its organ is the nerves; its objects are the primary qualities—hot, dry, moist and cold—and the qualities that follow from them, such as hard, soft, thick and thin. Philosophers raise many delightful questions about these five senses, their organs, objects and media, which I omit for brevity.