From The Search for Truth by Nicolas Malebranche

One need not imagine, as most philosophers do, that the mind becomes material when united with the body, and that the body becomes mind when it unites with the mind. The soul is not spread through all parts of the body, in order to give life and movement to it, as the imagination might have it; and the body does not become capable of sensation through its union with the mind, as our false and misleading senses seem to convince us. Each substance remains what it is, and as the soul is incapable of extension and movement, so the body is incapable of sensation and inclinations. The only alliance of mind and body known to us consists in a natural and mutual correspondence of the soul's thoughts with the brain traces, and of the soul's emotions with the movements of the animal spirits.

As soon as the soul receives some new ideas, new traces are imprinted in the brain; and as soon as objects produce new traces, the soul receives new ideas. It is not that it considers these traces, since it has no knowledge of them; nor that these traces include these ideas, for they have no relation to them; nor, finally, that the soul receives its ideas from these traces; for, as we shall explain in the third book, it is inconceivable that the mind receive anything from the body and become more enlightened by turning toward it, as these philosophers claim who would have it that it is by transformation to fantasms, or brain traces, per conversionem ad phantasmata, that the mind perceives all things. But that all takes place according to the general laws of the union of soul and body, which I shall also explain in the third book.

Likewise as soon as the soul wills that the arm be moved, it is moved, even though the soul does not know what it must do in order to move it; and as soon as the animal spirits are agitated, the soul is affected, even though it might not even know whether there are animal spirits in its body.

When I come to speak of the passions, I shall talk about the connection between the brain traces and the movements of the spirits, and that between the ideas and the emotions of the soul, for all the passions depend on them. Right now, I need only mention the connection between the ideas and the traces, and the connection of the traces with each other.

There are three very important causes of the connection of ideas with traces. The first, and the one the others presuppose, is nature, or the constant and immutable will of the Creator. There is, for example, a natural connection, independent of our will, between the traces producing a tree or a mountain we see and the ideas of tree or mountain, between the traces that produce in our brain the cry of a suffering man or animal and our understanding him to complain, between the expression of a man who threatens or fears us and the ideas of pain, strength, weakness, and even among the feelings of compassion, fear, and courage arising in us.

These natural connections are the strongest of all. They are generally similar in all men, and they are absolutely necessary for the preservation of life. This is why they do not depend at all upon our wills. For, if the connection of ideas with sounds and certain characters is weak, and quite different in different countries, it is because it depends, upon the weak and changeable will of men. And the reason why this connection depends upon it is that this connection is not absolutely necessary for living, but only for living as men, who should form a rational society among themselves.

It is this continuous and efficacious impression of the will of God on us that binds us so closely to one part of matter, and if this impression of His will should cease for but a moment, we would immediately be freed from our dependence upon the body and all the changes it undergoes, For I cannot understand how certain people imagine that there is an absolutely necessary relation between the movements of the spirits and blood and the emotions of the soul, A few tiny particles of bile are rather violently stirred up in the brain - therefore, the soul must be excited by some passion, and the passion must be anger rather than love. What relation can be conceived between the idea of an enemy's faults, or a passion of contempt or hatred, on the one hand, and the corporeal movement of the blood's parts striking against certain parts of the brain on the other? How can they convince themselves that the one depends on the other, and that the union or connection of two things so remote and incompatible as mind and matter could be caused and maintained in any way other than by the continuous and all-powerful will of the Author of nature?

... Now it appears to me quite certain that the will of minds is incapable of moving the smallest body in the world; for it is clear that there is no necessary connection between our will to move our arms, for example, and the movement of our arms. It is true that they are moved when we will it, and that thus we are the natural cause of the movement of our arms. But natural causes are not true causes; they are only occasional causes that act only through the force and efficacy of the will of God, as I have just explained.

For how could we move our arms? To move them, it is necessary to have animal spirits, to send them through certain nerves toward certain muscles in order to inflate and contract them, for it is thus that the arm attached to them is moved; or according to the opinion of some others, it is still not known how that happens. And we see that men who do not know that they have spirits, nerves, and muscles move their arms, and even move them with more skill and ease than those who know anatomy best. Therefore, men will to move their arms, and only God is able and knows how to move them. If a man cannot turn a tower upside down, at least he knows what must be done to do so; but there is no man who knows what must be done to move one of his fingers by means of animal spirits. How, then, could men move their arms? These things seem obvious to me and, it seems to me, to all those willing to think, although they are perhaps incomprehensible to all those willing only to sense.

But not only are men not the true causes of the movements they produce in their bodies, there even seems to be some contradiction (in saying) that they could be. A true cause as I understand it is one such that the mind perceives a necessary connection between it and its effect. Now the mind perceives a necessary connection only between the will of an infinitely perfect being and its effects. Therefore, it is only God who is the true cause and who truly has the power to move bodies. I say further (a) that it is inconceivable that God could communicate His power to move bodies to men or angels, and (b) that those who claim that our power to move our arms is a true power should admit that God can also give to minds the power to create, annihilate, and to do all possible things; in short, that He can render them omnipotent, as I shall show.

God needs no instruments to act; it suffices that He wills⁽¹⁾ in order that a thing be, because it is a contradiction that He should will and that what He wills should not happen. Therefore, His power is His will, and to communicate His power is to communicate the efficacy of His will. But to communicate this efficacy to a man or an angel signifies nothing other than to will that when a man or an angel shall will this or that body to be moved it will actually be moved. Now in this case, I see two wills concurring when an angel moves a body; that of God and that of the angel; and in order to know which of the two is the true cause of the movement of this body, it is necessary to know which one is efficacious. There is a necessary connection between the will of God and the thing He wills. God wills in this case that, when an angel wills this or that body be moved it will be moved. Therefore, there is a necessary connection between the will of God and the movement of the body; and consequently it is God who is the true cause of its movement, whereas the will of the angel is only the occasional cause.

But to show this still more clearly, let us suppose that God wills to produce the opposite of what some minds will, as might be thought in the case of demons or some other minds that deserve this punishment. One could not say in this case that God would communicate His power to them, since they could do nothing they willed to do, Nevertheless, the wills of these minds would be the natural causes of the effects produced. Such bodies would be moved to the right only because these minds willed them moved to the left; and the volitions of these minds would determine the will of God to act, as our willing to move the parts of our bodies determines the first cause to move them. Thus, all the volitions of minds are only occasional causes.

1. It is clear that I am speaking here about practical volitions, or those God has when He wills to act.