General idea of the imagination

In the previous book we discussed the senses. We sought to explain their nature and define precisely how they ought to be used; we identified the main and most common errors into which they lead us, and we sought to set proper limits to their power, so that we may trust them where they serve us, and fear them only if we fail to keep them within the bounds we have described. In this second book we shall treat of the imagination; the natural order of our inquiry requires it, for the connection between the senses and the imagination is so close that they ought not to be separated. As we shall see later, these two faculties differ only in degree, not in kind.

Here is the plan we shall follow: this work is divided into three parts. In the first, we explain the physical causes of the imagination’s irregularities and errors. In the second, we apply these causes to the most common mistakes of the imagination, and also examine what may be called the moral causes of these errors. In the third, we discuss how powerful imaginations can influence and spread their effects to others.

Although much of what follows may not be as novel as the explanations we have already given regarding the errors of the senses, it will be no less useful. Thoughtful people already recognize these errors and their sources; yet very few pause to reflect deeply upon them. I do not claim to instruct everyone; I seek to enlighten those who know little, and merely to remind those who know more — or rather, I write here chiefly to instruct and remind myself.

General idea of the imagination

We stated in Book One that the organs of sense are made up of fine filaments, which extend on one side to the outer parts of the body and the skin, and on the other toward the center of the brain. These filaments can be moved in two ways: either starting from the ends inside the brain, or from those on the body’s surface.

Whenever these filaments are disturbed, the soul perceives something. If the movement begins when external objects press upon the outer ends of our nerve‑filaments and is transmitted all the way to the brain, the soul feels and judges that what it perceives exists outside itself — in other words, it sees the object as present. But if only the inner ends of the filaments are set in motion by the flow of the animal spirits or by some other internal cause, the soul imagines and judges that what it perceives is not outside, but inside the brain — that is, it sees the object as absent. This is the essential difference between sensing and imagining.

We must note, however, that the brain’s fibers are moved far more strongly by impressions from external objects than by the flow of the animal spirits. This is why the soul is affected much more vividly by things it judges to be present and capable of causing pleasure or pain, than by the movements that produce images in the mind.

Yet it sometimes happens in people whose animal spirits are unusually active — due to fasting, lack of sleep, fever, or intense passion — that these spirits can stir the inner fibers of the brain with as much force as external objects. In such cases, they feel what ought only to be imagined, and believe they see before their eyes things that exist only in their own minds. This confirms what I have said: physically speaking, the senses and the imagination differ only in degree.

To give a clearer and more precise definition of the imagination, we must understand that every change occurring in the region of the brain where the nerves converge corresponds to a change in the soul. As we have already explained, whenever the flow of spirits alters the arrangement of the fibers in this area, a new perception arises in the soul. It necessarily feels or imagines something new; and the soul can never feel or imagine anything new without a corresponding change in the fibers of this same part of the brain.

Thus, the faculty of imagining — or the imagination itself — consists in nothing more than the power the soul possesses to form images of objects by producing changes in the fibers of what we may call the principal part of the brain. This region is linked to all parts of the body, and it is here that the soul is immediately united to the body, if we may speak in such terms.

The Imagination involves 2 faculties, one active and the other passive

This makes clear that the soul’s power to form images involves two distinct elements: one that depends on the soul itself, and another that depends on the body. The first is the activity and command of the will; the second is the response of the animal spirits, which trace these images, and of the brain fibers, upon which they are impressed. In this work, we use the word “imagination” to refer to either of these aspects interchangeably. We do not usually distinguish them by the terms “active” and “passive,” because the context of our discussion will make clear whether we mean the active imagination of the soul or the passive imagination of the body.

We shall not attempt to identify exactly which part of the brain this “principal region” is. First, because such precision is unnecessary for our purpose. Second, because the matter remains uncertain. And finally, because even if we were certain of its location, we could not prove it in a way that would convince others; and it is better not to speak of what cannot be demonstrated.

Whether, as Willis believed, the “common sense” resides in the two structures he calls the corpora striata, while the folds of the brain store the images of memory and the corpus callosum serves as the seat of the imagination; or whether, following Fernel, it is found in the membrane that covers the brain; or whether it lies in the pineal gland as Descartes maintained; or even in some other part yet to be discovered — we need not concern ourselves with the exact location. It is enough to accept that there is such a principal region, and that the core of Descartes’ system remains valid. Even if he was mistaken — as seems likely — in claiming that the soul is immediately joined to the pineal gland, this error does not undermine the fundamental principles of his theory, from which we can still draw all the insight needed to advance our understanding of human nature.

The general cause of changes occurring in the human imagination, and the purpose of book 2

Since the imagination consists simply in the soul’s power to form images of objects by impressing them, as it were, upon the fibers of the brain, the deeper and more distinct the traces left by the animal spirits — which are the marks of these images — the more vivid and clear the soul’s imagination will be.

Just as the depth, breadth, and sharpness of lines in an engraving depend on the force of the tool and the receptivity of the metal, so the clarity and depth of the traces left by the imagination depend on the strength of the animal spirits and the condition of the brain’s fibers. It is the variations in these two factors that account for almost all the differences we observe between one person’s mental capacity and another’s.

We can readily explain the great diversity of human temperament and mental ability by considering: on the one hand, whether the animal spirits are abundant or scarce, fast‑moving or sluggish, coarse or fine; and on the other, whether the brain fibers are delicate or dense, moist or dry, flexible or rigid, and how well suited they are to respond to the flow of the spirits.

It would be very useful for each person to consider all the possible combinations of these factors, and then compare them with the differences they observe between people. It is always more valuable and rewarding to train the mind to discover truth for itself, rather than letting it grow dull from inactivity and receiving only ideas already fully worked out and explained. Moreover, some of these distinctions are so subtle that one may sense and recognize them in oneself, yet find it impossible to describe them clearly to others.

Nevertheless, to explain as fully as possible these differences between minds, and to help each person understand more easily the causes of the changes they feel in themselves from time to time, it seems best to examine generally the reasons why the animal spirits and the brain fibers change. In this way, we will also uncover the causes of all changes in the imagination.

Human nature is seldom constant for long; everyone feels within themselves how changeable they are. We judge the same matter one way at one time and quite differently at another. In short, our life consists not only in the circulation of the blood, but also in a constant flow of thoughts and desires. We can scarcely use our time more profitably than by seeking to understand the causes of these inner changes, and thereby learning to know ourselves.