The proposition which was the occasion of all this discussion is very important and should be clearly established, for from it follows that every individual substance expresses the whole universe according to its way and under a certain aspect, or, so to speak, according to the point of view from which it is regarded; and that a succeeding condition is a consequence, whether free or contingent, of its preceding state as though only God and itself were in the world. Thus every individual substance or complete being is, as it were, a world apart, independent of everything else excepting God. There is no argument so cogent not only in demonstrating, the indestructibility of the soul, but also in showing that it always preserves in its nature traces of all its preceding states with a practical remembrance which can always be aroused, since it has the consciousness of or knows in itself what each one calls his me. This renders it open to moral qualities, to chastisement and to recompense even after this life, for immortality without remembrance would be of no value. This independence however does not prevent the inter-activity of substances among themselves, for, as all created substances are a continual production of the same sovereign Being according to the same designs and express the same universe or the same phenomena, they agree with one another exactly; and this enables us to say that one acts upon another because the one expresses more distinctly than the other the cause or reason for the changes,- somewhat as we attribute motion rather to a ship than to the whole sea; and this with reason, although, if we should speak abstractly, another hypothesis of motion could be maintained, that is to say, the motion in itself and abstracted from the cause could be considered as something relative. It is thus, it seems to me, that the interactivities of created substances among themselves must be understood, and not as though there were a real physical influence or dependence. The latter idea can never be distinctly conceived of. This is why, when the question of the union of the soul and the body, or of action and of passion of one spirit with regard to another created thing, comes into question, many have felt obliged to grant that their immediate influence one upon another is inconceivable. Nevertheless, the hypothesis of occasional causes is not satisfactory, it seems to me, to a philosopher, because it introduces a sort of continuous miracle as though God at every moment was changing the laws of bodies on the occasions when minds had thoughts, or was changing the regular course of the thinking of the soul by exciting in it other thoughts on the occasion of a bodily movement; and in general as though God was interfering otherwise for the ordinary events of life than in preserving each substance in its course and in the laws established for it. Only the hypothesis of the concomitance or the agreement of substances among themselves therefore is able to explain these things in a manner wholly conceivable and worthy of God. And as this hypothesis alone is demonstrative and inevitable in my opinion, according to the proposition which we have just established, it seems also that it agrees better with the freedom of reasonable creatures than the hypothesis of impressions or of occasional causes. God created the soul from the very start in such a manner that for the ordinary events it has no need of these interventions, and whatever happens to the soul comes from its own being, without any necessity, on its part, of accommodation in the sequence of events to the body, any more than there is of the body's accommodating itself to the soul. Each one follows its laws, the one acts freely, the other without choice, and they accord with one another in the same phenomena. The soul is nevertheless the form of its body, because it expresses the phenomena of all other bodies according to their relation to its own.

It may be surprising, perhaps, that I deny the action of one corporeal substance upon another, when this seems so evident, but, besides the fact that others have already done this, we must also consider that it is rather a play of the imagination than a distinct conception. If the body is a substance and not a mere phenomenon, like a rainbow, nor a being, brought together by accident or by accumulation, like a pile of stones, its essence cannot consist in extension and we must necessarily conceive of something which is called substantial form and which corresponds in some sort to the soul. I have been convinced of this, as it were, in spite of myself, after having held a very different opinion before. But, however much I may approve of the Schoolmen in this general and, so to speak, metaphysical accounting for the basis of bodies, I also hold to the corpuscular theory as it is used in the explanation of particular phenomena, and for these latter nothing is gained by applying the terms, forms and qualities. Nature must always be explained mathematically and mechanically, provided it be kept in mind that the principles or the laws of mechanics and of force do not depend upon mathematical extension alone but have certain metaphysical causes.

After all this I think that now the propositions contained in the abstract which was sent to you will appear not only more intelligible but perhaps tenable and more important than might have been thought at first.

X: Leibniz to Arnauld

Hanover, July 14, 1686.

Monsieur: