Yours very affectionately,

E.

XIII: Draft of the letter of Nov. 28- Dec. 8 to Arnauld

The hypothesis of concomitance is a consequence of the conception which I have of substance, for, in my opinion, the individual concept of a substance involves all that will ever happen to it, and it is in this that complete beings differ from those which are not complete. Now, since the soul is an individual substance it must be that its concept, idea, essence or nature involves all that will happen to it, and God, who sees it perfectly, sees there what it will do or endure forever and all the thoughts which it will have. Therefore, since our ideas are only the consequences of the nature of the soul and are born in it by virtue of its concept, it is useless to ask regarding the influence of another particular substance upon it. This aside from the fact that this influence would be absolutely inexplicable. It is true that certain thoughts come to us when there are certain bodily movements and that certain bodily movements take place when we have certain thoughts, but this is because each substance expresses the whole universe in its fashion and this expression of the universe which brings about a movement in the body is perhaps a pain in regard to the soul. It is customary to attribute the action to that substance whose expression is more distinct and which is called the cause, just as when a body is swimming in water there are an infinity of movements of the particles of water in such a way that the place which the body leaves may always be filled up in the shortest way. This is why we say that this body is the cause of the motion, because by its means we can explain clearly what happens. But if we examine the physics and the reality of the motion, it is quite as easy to suppose that the body is in repose and that all the rest is in motion conformably to this hypothesis, since every movement in itself is only relative, that is to say, is a change of position which cannot be assigned to any one thing with mathematical precision; but the change is attributed to that body by means of which the whole is most clearly explained, In fact, if we take all phenomena, great or small, there is only one single hypothesis which serves to explain everything clearly. We can therefore say, that, although this body is not an efficient physical cause of these effects, its idea is at least, so to speak, the final cause of them, or, if you prefer, a model cause *003 of them in the understanding of God; because, if we wish to ask what reality there is in motion we may imagine that God desires expressly to produce all the changes of position in the universe exactly the same as that ship was producing them while going through the water. Is it not true that it happens exactly in the same way, for it is not possible to assign any real difference? If we speak with metaphysical precision there is no more reason for saying that the ship presses upon the water in order to make that large number of circular movements because of which the water takes the place of the ship, than to say that the water itself exerts pressure to make all these circles and that it therefore causes the ship to move conformably. Unless we say, however, that God expressly desired to produce such a great number of movements so well fitted together, we do not give any real cause for it, and as it is not reasonable to have recourse to divine activity for explaining a particular detail, we have recourse to the ship, notwithstanding the fact that, in the last analysis, the agreement of all the phenomena of different substances comes about only because they are productions of the same cause, that is to say, of God. Therefore, each individual substance expresses the resolves which God made in regard to the whole universe. It is therefore for the same reason that pain is attributed to changes in the body, because thus we reach something distinct and this is enough for us to produce the phenomena or to prevent them. In order not to advance anything that is unnecessary, however, I say that we only think, and also that we produce only thoughts, and that the phenomena are only thoughts. As, however, all our thoughts are not effective and do not serve to produce for us others of a certain nature, and since it is impossible for us to work out the mystery of the universal connection between phenomena, we must pay attention by means of experience to those which have produced thoughts before, and this is the way the senses do and this is what is called external action, outside of us.