The Types of Human Senses

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

The human soul is united to the whole body.

Its principal seat is 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 around the seat of the soul, as explained 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.

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: the internal 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

However, we do have not 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

One of our internal senses is the natural appetite. These are made up of the nerves which extend to:

• the stomach
• the oesophagus
• the fauces
• the other internal parts that are subservient to our natural wants.

The other internal sense embraces all the passions and affections, or emotions of the mind.

• Examples are joy, sadness, love, hate, and the like.

This depends on the nerves which extend:

• to the heart and
• to the parts around the heart.

These are exceedingly small.

For example, when the blood is 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.

These movements 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.

• Instead, it causes the animal spirits to pass from the brain to the muscles in which these nerves are inserted.
• This then dilates the orifices of the heart.
• This causes these small nerves to move to excite the sensation of joy.

Thus, when we receive news, the mind first of all judges of it.

• If the news is good, it rejoices with that intellectual joy which is independent of any emotion of the body.

The Stoics allowed their wise man to have such intellectual joy [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 around the heart.*

• There they excite the motion of the small nerves.
• This causes another motion in the brain which affects the mind with the sensation of animal joy.

On the same principle, sometimes the blood is so thick that it flows sparingly into the ventricles of the heart and there is not sufficiently dilated.

• This excites in the same nerves a motion quite different from that of joy.
• this movement is communicated to the brain which then gives to the mind the sensation of sadness, even if 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.

There is the widest difference between:

• these passions
• the distinct thoughts which we have of what should 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.

• But their sources are the nerves of the stomach, fauces, and other parts, not the heart.

These are totally 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 they are named appetites because this will or appetition almost always accompanies them.

191. The external senses: Touch

We commonly count 5 external senses 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.

The nerves are either moved or hindered from their ordinary motion in many diverse modes.

• These lead to diverse sensations excited in the mind.

A sensation of pleasure arises when these nerves are moved a little more powerfully than usual, but without hurting our body.

• This is naturally agreeable to the mind because it testifies to it the powers of the body of being affected by the action causing this titillation, without being hurt.

But if this action is strong enough to hurt our body in any way, this gives to our mind the sensation of pain.

This is why corporeal pleasure and pain arise from causes nearly alike, even if they are sensations of quite an opposite character.

192. Taste

2. The nerves scattered over the tongue are diversely moved by bodies on it, floating in the saliva in the mouth.

This causes sensations of diverse tastes, according to the diversity of the shapes in these particles.

193. Smell

3. These 2 nerves are moved by the particles of terrestrial bodies flying in the air

Those particles must be subtle enough to:

• enter into the nostrils
• penetrate and enter the pores of the bone we call the spongy
• 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.

These vibrations give rise, according to their diversity, to the sensations of the different sounds.

195. Sight

5. The extremities of the optic nerves compose the coat in the eyes called the retina.

These are not moved by the air nor by any terrestrial object, but only by the air-aether globules, whence we have the sense of light and colours.

I have explained this in the Dioptrics and Meteors.