Regarding the difficulty of admitting into space forces non-extended and consequently having no relation to space, I grant that it is very serious. It cannot be raised, however, by those who consider the soul as a non-extended force and as an individual substance; for they are obliged to recognize that it is in space although in its essence it has no relation to space; there is, therefore, for them no contradiction in holding that a simple force is in space. If, on the other hand, it be denied that the soul is in space, that it is in the body, and even that it is in a certain part of the body, is it not clear that this would be attributing to the soul a character which is true only of God? To be sure, those who consider the soul as a divine idea, an eternal form temporarily united to an individual, might speak thus. Thus regarded, with the idealists or with Spinoza, the soul is not in space. But if the soul is represented as an individual and created substance, how can it be thought of except as in space and in the body to which it is united? Still more, therefore, in the case of monads will we be obliged to admit that they may be in space and then, as we have seen, the appearance of extension is explained without difficulty.
If, now, instead of admitting the reality of space we hold with Leibniz or with Kant that it is ideal, the system of monads offers no longer any serious difficulty, except from the point of view of those who deny the plurality of individual substances. In any case Euler’s objection evidently loses its force.
Another difficulty raised against the monadology is that it effaces the distinction between the soul and the body. This difficulty seems to me like the preceding one to be merely apparent. Because in every hypothesis, the essential distinction between the body and the soul is that the body is a composite, while the soul is simple. In order to prove that the soul is not extended the proof is offered that it is not a composite, while the body on the contrary is. Now in Leibniz’s hypothesis also, the body is only a composite, only an aggregation of simple elements. What difference does the nature of the elements make in this case? It is the whole, it is the aggregation which we contrast with the soul; and in Leibniz’s hypothesis, quite as well as in that of Descartes, the body as an aggregation is wholly incapable of thought.
Some one will reply: “granting all that, the elements are nevertheless single and indivisible like the soul itself and they are therefore of the same nature as the soul—they are souls themselves.” This last consequence is very incorrectly drawn, however.
What is meant by the words: “of the same nature”? Does it mean that the monads which compose the body are feeling, thinking, willing beings? Leibniz never said such a thing. What is the basis for affirming that the particles of my body are thinking substances? Let us look at the semblance they have to the soul. Doubtless they are like it single and indivisible substances. But what difficulty does it introduce to admit that the soul and body have common attributes? The atoms, for instance, have they not in common with the soul, existence, indestructibility, self-identity? And does the argument of the identity of the ego in contrast with the changing nature of organized matter, cease to be valid, because the atom is quite as self-identical as the soul? Indeed the indestructibility of the atom is used as an analogy to establish the indestructibility of the soul. If this common character does not prevent their being distinguished, why should their being distinguished be more difficult when they have in common a character essential to all substance, namely, the attribute of activity?
Furthermore, if the atoms of the substance, which constitutes the universe, are indivisible units, the power of thinking is not inconsistent with their conception. They may be thinking substances, and it cannot be denied that in this system a monad may become, if God wishes it, a thinking soul. If on the one hand it is not impossible, there is no way, on the other hand, of proving that it may be so. Why may there not be several orders of monads which are unable to pass from one class to another? Why may there not be monads having merely mechanical properties; others of a higher order, containing the principle of life, like plant souls; still higher sensitive souls; and finally free and intelligent souls endowed with personality and immortality? Leibniz’s system is no more opposed than any other to these orders.
If, however, by a bolder hypothesis, the possibility of a monad’s passing from one order to another be admitted, there would still be nothing here degrading to the true dignity of man, for, after all it must be recognized that the human soul in its first state is hardly anything more than a plant-soul which lifts itself by degrees to the condition of a thinking soul. Therefore there will be no contradiction in admitting that every monad contains potentially a thinking soul. Should such a hypothesis be repugnant, I still maintain that the monadological system does not force one to it, since monadism quite as well as the popular atomism can admit a scale of substances essentially distinct from one another.
Another objection which the Leibnizian excites, and one which Arnauld does not fail to raise in one of his letters, is that the system of monads weakens the argument of a first mover, since it implies that matter can be endowed with active force and consequently with spontaneous motion. Leibniz does not meet this objection in a convincing manner and says merely that recourse must be had to God to explain the co-ordination of movements. This, however, avoids the point, for the co-ordination has no relation to the argument of the first mover, only to that of the ordering and of the arrangement which is a wholly different matter. We may, however, remark that Leibniz, in order to establish the reality of the force in corporeal substance, much more frequently uses the fact of resistance to motion, than that of the so-called spontaneous motion. For instance, one of his principal arguments is that a moving body, when it comes in contact with another, loses motion in proportion to the resistance which the other opposes to it, and this is what he calls inertia. It is evident, therefore that if a substance in repose reveals itself by its resistance to motion, the argument of the first mover, far from being weakened is, on the contrary, strengthened.
Besides this, even if a spontaneous disposition to movement, be admitted in the elements of bodies, yet experience compels us to recognize that this disposition passes over into action only upon the excitation of an exterior action because we never see a body put in motion except in the presence of another. The actual indifference to movement and to repose, which at the present time is called, in mechanics, inertia, must always be admitted, whether we posit in the body a virtual disposition to movement or whether, on the contrary, the body be considered as absolutely passive; in either case there must be a cause determining the motion; it is not necessary that this first cause produce everything in the body moved, and that it should be in some sort the total cause of the motion; sufficient is it for it to be the complementary cause as the Schoolmen used to say.
Furthermore inertia must not be confounded with absolute inactivity. Leibniz showed admirably that an absolutely passive substance would be a pure nothing; that a being is active in proportion as it is in existence; in a word, that to be and to act are one and the same thing. From the fact, however, that a substance is essentially active, it does not necessarily follow that it is endowed with spontaneous motion, for the latter is only a special mode of activity and is not the only one. For example, resistance, or impenetrability, is a certain kind of activity, but is not motion. They are mistaken, therefore, who think that the theory of active matter does away with a first cause for motion, because even if motion be essential to matter, we will still have to explain why no portion of matter is ever spontaneously in motion.
In short, according to Leibniz, every being is essentially active. That which does not act does not exist; quid non agit non existit. Now, whatever acts is force; therefore, everything is force or a compound of forces. The essence of matter is not, as Descartes thought, inert extension, it is action, effort, energy. Furthermore the body is a compound and the compound presupposes a simple. The forces, therefore, which compose the body are simple elements, unextended—incorporeal atoms. Thus the universe is a vast dynamism, a wise system of individual forces, harmoniously related under the direction of a primordial force, whose absolute activity permits the existence outside of itself of the appropriate activities of created things, which it directs without absorbing them. This system, therefore, may be reduced to three principal points: (1) it makes the idea of force predominate over the