I confess, M., that my ideas on the laws of motion are not clear and distinct enough to enable me to pass judgment upon the difficulty which you have brought up against the Cartesians. The one who replied to you is the Abbe Catelan who has a good mind and is a good geometer; since I left Paris I have not had much intercourse with the philosophers of that country. Inasmuch, however, as you have decided to reply to this Abbe, and as he will perhaps wish to defend his position, it is to be hoped that these discussions will so clear up the difficulty that it will be possible to know which side to take.

I thank you very much for the desire you show to know how I am. I am thankful to say that I am very well for my age, only I had a very bad cold at the beginning of the winter. I am very glad that you are thinking of completing your arithmetical machine. It would be a pity if so fine an invention were lost. I desire greatly, however, that the intention in regard to which you wrote a word to the Prince, who has so much affection for you, may not remain without its effects, for there is nothing towards which a wise man should work with more care and with less of delay than towards what has to do with his salvation. I am, Monsieur,

Your very humble and obedient servant.

A. ARNAULD.

XVII: Leibniz to Arnauld

Gottingen, April 30, 1687.

Monsieur:

Since I regard your letters as personal benefactions to me and as sincere marks of your liberality, I have no right to ask for them and consequently your reply is never too late. However agreeable and useful they may be to me, I take into consideration what you owe to the public weal and thus suppress my desires. Your criticisms are always instructive and I will take the liberty to go through them in order.

I do not think that there is any difficulty in my saying that "the soul expresses more distinctly, other things being equal, that which pertains to its own body"; since it expresses the whole universe in a certain sense according to the special relation of other bodies to itself, it is not able to express all things equally, otherwise there would be no distinction between souls, but it does not follow from this that the soul should perceive perfectly whatever goes on in the parts of its body, since there are degrees of relationship between these parts themselves and these parts are no more equally expressed than are external things. The greater distance of the latter is made up for by the smallness or by some other hindrance in the internal parts. Thales saw the stars though he did not see the ditch which was at his feet.

The nerves and membranes are for us the parts which are more sensitive than the others and it is, perhaps, only through them that we perceive what seems to happen to the others, because the movements of the nerves, or of the liquids in them, imitate the impressions better and confuse them less, and the most distinct expressions of the soul correspond to the most distinct impressions of the body. Metaphysically speaking, it is not the nerves which act upon the soul, but the one represents the state of the other through the spontaneous relation; it must also be remembered that too many things take place within our bodies to be all separately perceived; we feel only a certain result, to which we are accustomed, and we are not able to distinguish the elements that are involved, because of their multitude. Just as when we hear from afar the sound of the sea, we do not distinguish what each wave does, although each wave has its effect upon our ears. When a striking change happens in our body, we notice it at once and more clearly than the changes outside, which are not accompanied by any special change in our organs.

I do not say that the soul knows the pricking before it has the sense of pain, except as it knows or expresses confusedly all things in accordance with the principles already established. This expression, however, although obscure and confused, which the soul has in advance of the future, is the real cause of that which happens to it and of the clearer conception which it will have later when the obscurity shall have worked out. The future state is only a consequence of the preceding.

I said that God created the universe in such a way that the soul and the body, each acting according to its laws, agree in their phenomena. You think, M., that this coincides with the hypothesis of occasional causes. Were this so I should not be sorry, and I am always glad to find those who hold my positions. I see, however, the reason for your thinking this. You are of the opinion that I would not say a body can move itself, and, the soul not being the real cause either of the motion of the arm or of the body, therefore the cause must be God. My opinion, however, is different. I hold that whatever reality there is in the state which is called motion, may issue quite as well from the bodily substance as the thought and will proceed from the spirit. Everything happens to each substance in consequence of the first state which God gave to it in creating it, and putting aside extraordinary interventions the ordinary agreement consists only in the conservation of the substance itself conformably to its preceding state and to the changes which it carries in itself. Nevertheless, we have the right to say that one body pushes another; that is to say, that one body never begins to have a certain tendency excepting when another which touches it loses proportionately, according to the constant laws which we observe in phenomena; and since movements are rather real phenomena than beings, a movement as a phenomenon is in my mind the immediate consequence or effect of another phenomenon, and the same is true in the minds of others. The condition of one substance, however, is not the immediate consequence of the condition of another particular substance.