The Flow from One Idea to Another is Caused by Animal Spirits

I establish a general maxim in this science of human nature: wherever there is a close relation between 2 ideas, the mind tends to mistake them and use the one for the other in all its reasonings.

What causes this?

I mentioned resemblance, contiguity, and causation, as principles of union among ideas, without examining their causes so that I could prosecute my first maxim.

It would be easier to dissect the brain and observe why the animal spirits run through all those relations when it thinks of any idea, and rouses up the other ideas related to it.*

The mind is endowed with a power of exciting any idea it pleases, whenever it dispatches the spirits into that region of the brain where the idea is placed.

These spirits always excite the idea when they:

• run precisely into the proper traces, and
• rummage that cell which belongs to the idea.

Their motion:

• is seldom direct, and
• naturally turns a little to the one side or the other.

This is why the animal spirits, falling into the contiguous traces, present other related ideas in lieu of that idea which the mind desired to survey at first.

We are not always sensible of this change.

Continuing the same train of thought, we make use of the related idea, which is presented to us, and employ it in our reasoning, as if it were the same with what we demanded.

This is the cause of many mistakes and sophisms in philosophy.

Resemblance is the most fertile source of error of the 3 relations above-mentioned.

Resembling ideas are related together.*

The mind’s actions which we employ in considering them are so similar, that we are unable to distinguish them.

We therefore take the one for the other.

Causation and contiguity may also create mistakes.

This is seen in poets and orators as they use use words for ideas.

Words are so commonly closely connected, that the mind easily mistakes them.

The idea of distance is not visible nor tangible.

This is why we substitute the idea of a distance with the idea of space.

Causation and resemblance creates this mistake.

This is because the mind converts the invisible and intangible distance into the visible and tangible distance, as space.

In this respect, the invisible distance is a kind of cause, while the similarity of their way of affecting the senses, and reducing every quality, creates the resemblance.

I can now answer all the objections from metaphysics or mechanics.

A vacuum is a space without matter.

The frequent disputes about a vacuum do not prove the reality of the idea of a vacuum.

Men most commonly deceive themselves in this, especially when there is another related idea presented which can become the cause of their mistake.

We may almost make the same answer to the second objection, derived from the conjunction of the ideas of rest and annihilation.

When everything is annihilated in the room and the walls stay immovable, the room must be conceived in the same way as at present.

Presently, the air that fills it is not an object of the senses.

This annihilation gives the eye that fictitious distance discovered by:

• the different parts of the organ that are affected.
• the degrees of light and shade
• the feeling from a motion in the hand.

We should not search any farther.

On whichever side we turn this subject, these are the only impressions that such an object can produce after the supposed annihilation.

Impressions can only give rise to resembling ideas.

Since a body interposed between two other bodies may be annihilated without producing any change on either hand of it, we can easily conceive how any change may be:

• created anew
• yet produce as little alteration.

The motion of a body has much the same effect as its creation.

The distant bodies are no more affected in the one case, than in the other.

This satisfies the imagination.

It proves there is no repugnance in such a motion.

Afterwards, experience persuades us that:

• two bodies situated in the above manner, really have such a capacity of receiving body between them, and
• there is no obstacle to the conversion of the invisible and intangible distance into one that is visible and tangible.

However natural that conversion may seem, we cannot be sure it is practical before experiencing it.

These answer the 3 objections above-mentioned.