In addition I cannot approve the custom of those who have recourse to their ideas, when they are at the end of their proofs, and who abuse the principle that every clear and distinct conception is good. For I hold that we must possess the criteria of distinct knowledge. And seeing that we often think without ideas, employing in place of the ideas in question, characters whose signification we wrongly suppose ourselves to know, and thus form impossible chimeras, therefore I hold that the criterion of a true idea is that its possibility can be proved, whether a priori in conceiving its cause or reason, or a posteriori when experience enables us to know that it is actually found in nature. This is why I consider definitions to be real when it is known that the defined is possible; otherwise they are only nominal and cannot be trusted; for if by chance the thing defined implies contradictions, two contradictories can be deduced from the same definition. It is for this reason that you had good cause to insist against Father Malebranche that a distinction must be made between true and false ideas, and that too much confidence must not be placed in the imagination under the pretext of a clear and distinct intellection.

I know no one who is better able than yourself to examine this class of thoughts, particularly those whose consequences lead into theology; few people having the necessary penetration and the broad enlightenment which is called for; and few people having that fairness which you have now displayed toward me. I therefore pray God to lengthen your life and not to deprive us too soon of an ally whose like will not be easily found again.

I am yours, sincerely, Monsieur,

XI: Arnauld to Leibniz

Sept. 28, 1686.

I thought, M., that I might make use of the liberty which you gave me to take my time in replying to your kindness; and therefore I have put it off, until I had completed a work which I had commenced. I have been a gainer in doing you justice, for there was never anything more honorable or more gracious than the manner in which you received my excuses. So much was not called for to make me resolve to acknowledge in good faith that I am satisfied with the manner in which you have explained what was startling to me at first, regarding the concept of the individual nature. For no man of honor should have any difficulty in accepting a truth as soon as it is made known to him. I have been above all struck by this argument, that in every affirmative true proposition, necessary or contingent, universal or singular, the concept of the attributes is comprised in some way in that of the subject. Predicatum inest subjecto.

There remains for me only the difficulty in regard to the possibility of things and in regard to this way of conceiving of God as though he had chosen the universe, which he created, out of an infinity of other possible universes which he saw at the same time and which he did not choose to create. But as this has nothing to do properly with the concept of the individual nature, and as I should have to meditate at too great length in order to make clear what I think about it or rather what I find to object to in the thoughts of others, because they do not seem to me to do justice to God's power, you will permit me to pass over this subject.

I would prefer to ask you to clear up two things which I find in your last letter. They seem to me important, but I do not understand them very well.

The first is as to what you mean by "the hypothesis of the concomitance and of the agreement of substances among themselves." You claim that by this means, that which happens in the union of the soul and the body and in the action or the passion of a mind with respect to any other created thing, can be explained. I cannot understand what you say in explaining this thought, which, according to you, agrees neither with those who think that the soul acts physically upon the body and the body upon the soul, nor with those who think that God alone is the physical cause of these effects, and that the soul and the body are only the occasional causes. You say, "God created the soul in such a way that for the ordinary events it has no need of these changes, and that which happens to the soul arises from its own being without its having to agree with the body in what results, any more than the body does with the soul. Each one follows its laws. The one acting with freedom and the other without choice, they fit in together, one with another, in the same phenomenon." Examples will enable you to make your thought clearer: some one wounds my arm. With regard to my body, this is only a bodily motion but my soul at once has a feeling of pain which it would not have if this had not happened to my arm. The question is, what is the cause of this pain? You deny that my body has acted upon my soul, and that God, on the occasion of this which happened to my arm, immediately produced in my soul the feeling of pain. It must be, therefore, that you think that it is the soul which has formed this feeling in itself and this must be what you mean when you say that, "What happens in the soul on the occasioning of the body arises from its own being." St. Augustine was of this opinion because he thought that bodily pain was nothing else than the grief which the soul had when its body was ill-affected. But what reply can be made to those who object that the soul must therefore have known that its body was ill-affected before it could become sorrowful, while in fact it seems to be the pain which informs the soul that the body is injured.