The Types of Human Senses

¹⁸⁹ What perception (SENSUS) is, and how we perceive.

The human soul is united to the whole body. But it has, nevertheless, its principal seat in the brain, where alone it understands, imagines, and perceives.

This done by the medium of the nerves, which are extended like threads from the brain to all the other members. These are so connected that we can hardly touch any one of them without moving the extremities of some of the nerves spread over it.

This motion passes to the other extremities of those nerves which are collected in the brain round the seat of the soul, [Footnote: *** FOOTNOTE NOT VISIBLE IN PAGE IMAGE (#98, Text p 195)] as I have already explained with sufficient minuteness in Chapter 4 of the Dioptrics.

But the movements which are thus excited in the brain by the nerves variously affect the soul or mind, which is intimately conjoined with the brain, according to the diversity of the motions themselves. And the diverse affections of the mind or thoughts that immediately arise from these motions, are called perceptions of the senses (SENSUUM PERCEPTIONES), or, as we commonly speak, sensations (SENSUS).

¹⁹⁰ The distinction of the senses; and, first, of the internal, that is, of the affections of the mind (passions), and the natural appetites.

The varieties of these sensations depend on:

1. The diversity of the nerves themselves
2. The movements that are made in each nerve.

We have not, however, as many different senses as there are nerves. We can distinguish but 7 principal classes of nerves:

• 2 are internal senses
• 5 are the external senses

The nerves which extend to the stomach, the oesophagus, the fauces, and the other internal parts that are subservient to our natural wants, constitute one of our internal senses. This is called the natural appetite (APPETITUS NATURALIS).

The other internal sense, which embraces all the emotions (COMMOTIONES) of the mind or passions, and affections, as joy, sadness, love, hate, and the like, depends on the nerves which extend to the heart and the parts about the heart. These are exceedingly small.

For example, when the blood happens to be pure and well tempered, it dilates in the heart more readily and strongly than usual.

• This enlarges and moves the small nerves scattered around the orifices.
• This produces a corresponding movement in the brain, which affects the mind with a natural feeling of joy.

As often as these same nerves are moved in the same way, although this is by other causes, they excite in our mind the same feeling (sensus, sentiment).

Thus, the imagination of the enjoyment of a good does not contain in itself the feeling of joy. But it causes the animal spirits to pass from the brain to the muscles in which these nerves are inserted.

Thus, dilating the orifices of the heart, it also causes these small nerves to move in the way appointed by nature to afford the sensation of joy.

Thus, when we receive news, the mind first of all judges of it, and if the news be good, it rejoices with that intellectual joy (GAUDIUM INTELLECTUALE) which is independent of any emotion (COMMOTIO) of the body, and which the Stoics did not deny to their wise man [although they supposed him exempt from all passion].

But as soon as this joy passes from the understanding to the imagination, the spirits flow from the brain to the muscles that are about the heart, and there excite the motion of the small nerves, by means of which another motion is caused in the brain, which affects the mind with the sensation of animal joy (LAETITIA ANIMALIS).

On the same principle, when the blood is so thick that it flows but sparingly into the ventricles of the heart, and is not there sufficiently dilated, it excites in the same nerves a motion quite different from the preceding, which, communicated to the brain, gives to the mind the sensation of sadness, although the mind itself is perhaps ignorant of the cause of its sadness.

All the other causes which move these nerves in the same way may also give to the mind the same sensation. But the other movements of the same nerves produce other effects, as the feelings of love, hate, fear, anger, etc., as far as they are merely affections or passions of the mind.

In other words, as far as they are confused thoughts which the mind has not from itself alone, but from its being closely joined to the body, from which it receives impressions; for there is the widest difference between these passions and the distinct thoughts which we have of what ought to be loved, or chosen, or shunned, etc., [although these are often enough found together].

The natural appetites, as hunger, thirst, and the others, are likewise sensations excited in the mind by means of the nerves of the stomach, fauces, and other parts, and are entirely different from the will which we have to eat, drink, [and to do all that which we think proper for the conservation of our body]; but, because this will or appetition almost always accompanies them, they are therefore named appetites.

¹⁹¹ The external senses; and first of touch.

We commonly reckon the external senses five in number, because there are as many different kinds of objects which move the nerves and their organs, and an equal number of kinds of confused thoughts excited in the soul by these emotions.

1. The nerves terminating in the skin can be touched through this medium by any terrene objects, and moved by these wholes, in one way by:

• their hardness
• their gravity
• their heat
• their humidity, etc.

In as many diverse modes as they are either moved or hindered from their ordinary motion, to that extent are diverse sensations excited in the mind, from which a corresponding number of tactile qualities derive their appellations.

Besides this, when these nerves are moved a little more powerfully than usual, but not nevertheless to the degree by which our body is in any way hurt, there thus arises a sensation of titillation, which is naturally agreeable to the mind, because it testifies to it of the powers of the body with which it is joined, [in that the latter can suffer the action causing this titillation, without being hurt].

But if this action be strong enough to hurt our body in any way, this gives to our mind the sensation of pain. And we thus see why corporeal pleasure and pain, although sensations of quite an opposite character, arise nevertheless from causes nearly alike.

¹⁹² Taste

2. The other nerves scattered over the tongue are diversely moved by the particles of the same bodies, separated from each other and floating in the saliva in the mouth, and thus cause sensations of diverse tastes according to the diversity of figure in these particles.

¹⁹³ Smell

3. 2 nerves also or appendages of the brain, for they do not go beyond the limits of the skull, are moved by the particles of terrestrial bodies, separated and flying in the air, not indeed by all particles indifferently, but by those only that are sufficiently subtle and penetrating to enter the pores of the bone we call the spongy, when drawn into the nostrils, and thus to reach the nerves. From the different motions of these particles arise the sensations of the different smells.

¹⁹⁴ Hearing

4. There are 2 nerves within the ears, so attached to 3 small bones that are mutually sustaining.

The first of these rests on the small membrane that covers the cavity we call the tympanum of the ear, that all the diverse vibrations which the surrounding air communicates to this membrane are transmitted to the mind by these nerves, and these vibrations give rise, according to their diversity, to the sensations of the different sounds.

¹⁹⁵ Sight

5. The extremities of the optic nerves, composing the coat in the eyes called the retina, are not moved by the air nor by any terrestrial object, but only by the globules of the second element, whence we have the sense of light and colours: as I have already at sufficient length explained in the Dioptrics and treatise of Meteors.

[Footnote: In the French this section begins, “Finally, sight is the most subtle of all the senses,” etc.]