The Senses Depend on 6 Circumstances

33. The senses depend on 6 circumstances.

1. The origin of the action originates.

This opens certain tubes into which the spirits first enter.

2. The efficacy and other qualities of that action.

3. The disposition of the filaments composing the substance of the brain.

4. The inequality of strength and efficacy that spirit particles can possess.

5. The different positions of external members.

6. The convergence of multiple actions that simultaneously affect the sense.

If, for example, object A, B, C acted on a sense other than sight, it would open different tubes in the inner surface of the brain than those noted as 2, 4, 6.

However, if the object were closer or farther away, or in a different position with respect to the eye than it currently is, it could open the same tubes but they would be arranged differently than they are at present.

This would direct the spirits to different locations than to a, b, c.

And so on for the rest.

As for the diversity of qualities of the actions that open those tubes, it is also clear that they would open them differently according to their diversity.

This difference alone is enough to alter the course of the spirits in the brain.

For example, if object A, B, C, is red, that is, if it acts on the eye 1, 3, 5, as is required for the sensation of the red color, and if that object further has the shape of an apple or another fruit: it must be considered that it will open the tubes 2, 4, 6 in a certain way.

This will cause the brain particles near N to press slightly more than usual against each other, while those near O will move away from each other, so that the spirits entering through those tubes 2, 4, 6 are directed from N through O towards P.

If the object A, B, C were of a different color and shape, precisely those filaments around N and O would not be the cause of the direction of the spirits entering through 2, 4, 6, but other nearby filaments would divert them elsewhere.

However, if the heat of fire A, which is close to hand B, were moderate, it must be considered that the way it opens the tubes 7 is the reason why the parts of the brain around N are pressed, while those around O are slightly more dilated than usual.

Thus, the spirits proceeding from tube 7 would tend from O to P.

But assuming that the fire burns the hand, its action opens the tubes 7 so much that the spirits flowing into it have the force to tend further in a straight line than up to N, up to r, where the pushing brain particles encountered in their transit press them in such a way that they are repelled through them towards S.

And so on for the others.

As for the disposition of the filaments composing the substance of the brain: it is either acquired or natural. The acquired one depends on all other circumstances that alter the course of the spirits.

The natural disposition of these filaments

God has so arranged these filaments so that their channels and pores can convey the spirits, agitated by a peculiar force, into all the nerves to which they should tend.

In this way, the spirits can excite the same motions in this bodily machine such as those arising from our instinct.

For example, when fire A burns hand B, it causes the spirits that enter tube 7 and go towards O. They find two principal pores and ducts, o, r and o, s.

o, r, conveys them into all the nerves that move the external members that is needed to avoid the fire by:

• retracting the hand, arm, or the entire body
• moving the head around and direct the eyes towards the fire

But through another duct, o, s, the spirits pass into all those nerves which serve internal movements, similar to those which follow pain in us. These contract the heart, agitate the liver, and similar others.

Likewise, into those nerves which can produce those external movements by which these internal movements are signified: as into those which induce tears, which furrow the brow and cheeks, and which prepare the voice for wailing.

But if hand B were not burnt by fire A but only moderately warmed, it would cause the same spirits that enter through tube 7 not to tend towards O anymore, but towards P, where they would again find pores arranged so that they could pass through them into all the nerves serving motions corresponding to this action.

I have not without reason distinguished two pores, o, r and o, s. Figure XLIX.

Namely, to remind you that almost always two kinds of motions arise from any action.

For external movements are excited, by which we pursue what we desire and avoid what is harmful.

The internal ones we commonly call “the principles of passions”. These dispose the heart and liver, and all the other organs on which the temperament of the blood, and therefore of the spirits, depends on. Those spirits they generate then become suited to excite external movements as required.

Different qualities of spirits are caused by one of those circumstances which changes their course.

For example, if some evil is to be avoided or overcome or repelled by force, to which the choleric passion inclines us, the spirits cannot but be agitated more unevenly and more vigorously than usual.

Conversely, if something must be avoided or borne patiently by dissimulation, to which fear inclines, a smaller quantity of spirits and less vehemence is required.

To produce this effect, the heart must be contracted, and the spirits must be spared, to preserve them for this work.

Similar reasoning must be applied analogically to other passions.

Other external movements do not lead to avoiding evil or obtaining good.

Examples are laughter or weeping.

These are merely testimonies of certain passions.

These only occur if some occasion arises. This is because the nerves through which the ingress of spirits occurs to create them are closely related to the nerves through which the spirits are received to excite passions, as is evident from anatomy.

How do the different qualities of spirits have the power to change their courses?

This occurs especially when they are determined from elsewhere only temporarily or not at all.

For example, if the nerves of the stomach are affected in the way that they must be moved to stimulate hunger, and in the meantime nothing edible or drinkable presents itself to any sense or memory;

the spirits which that action introduces into the brain through tubes 8 will locate themselves where they will find more pores arranged in such a way that they can be conveyed through them into all the nerves indiscriminately, by which our machine can seek or achieve a certain object: so that only the inequality of these particles can derive them into these nerves rather than into others.

If stronger particles of spirits are conveyed to certain nerves, but immediately afterwards other stronger ones rush forth to the nerves opposite to them, the machine will imitate the motions, Those which occur to us when we hesitate or doubt about something.

Similarly, if the action of fire A lies midway between those which can lead the spirits to O and to P, that is, between those which produce pain and pleasure:

Only their multiple inequalities, inherent in themselves, suffice to determine them towards one or the other, as the action is pleasing to us when we are in a cheerful mood, often displeasing when we are tormented by melancholy.

Hence, the explanation must be sought in those things which have been said about temperaments or inclinations, both natural and acquired, insofar as they depend on the difference in spirits.

The different positions of external limbs

Changing the pores through which the spirits are immediately conveyed into the nerves cause the limbs to momve.

For example, if when fire A burns hand B, the head would incline to the left, as it is now inclined to the right, the spirits would flow clearly, just as they now pass from 7 to N, then to O, and from there to r and s.

But from r, when it should pass to X, where I suppose it should tend, to turn the head to the right hand, they would go to Z: where, according to what I assume, they should tend if the head were turned to the left.

Because that position of the head, which is now the cause that the filaments of the substance of the brain, which are around X, are much looser and can be more easily separated from each other than those which are around Z: changing that position would produce the contrary effect: for those around Z would then be looser, while those around X would be very tense and tight.

This is how a single action without any change in itself can move one foot, and then the other foot as progress.

It is done when the spirits pass through only one pore, whose end is arranged differently, leading them into other nerves when the left foot has progressed farther than the right.

This is how respiration and similar other movements (which do not depend on any idea) work.