If we, like Descartes, assume independent substances, no causal nexus is conceivable; for this presupposes an influence, a bearing of the one upon the other, and in this way the other is not a substance. “Therefore there remains only harmony, a unity which is in itself or implicit. The monad is therefore simply shut up in itself, and cannot be determined by another; this other cannot be set into it. It can neither get outside itself, nor can others get inside it.”(8) That is also Spinoza's way of regarding matters: each attribute entirely represents the essence of God for itself, extension and thought have no influence on each other.
In the third place, “however, these monads must at the same time have certain qualities or determinations in themselves, inner actions, through which they are distinguished from others.
There cannot be two things alike, for otherwise they would not be two, they would not be different but one and the same.”(9) Here then Leibnitz's axiom of the undistinguishable comes into words.
What is not in itself distinguished is not distinguished. This may be taken in a trivial sense, as that there are not two individuals which are alike. To such sensuous things the maxim has no application, it is prima facie indifferent whether there are things which are alike or not; there may also be always a difference of space. This is the superficial sense, which does not concern us. The more intimate sense is, however, that each thing is in itself something determined, distinguishing itself from others implicitly or in itself. Whether two things are like or unlike is only a comparison which we make, which falls within our ken. But what we have further to consider is the determined difference in themselves. The difference must be a difference in themselves, not for our comparison, for the subject must have the difference as its own peculiar characteristic or determination, i.e., the determination must be immanent in the individual. Not only do we distinguish the animal by its claws, but it distinguishes itself essentially thereby, it defends itself, it preserves itself. If two things are different only in being two, then each of them is one; but the fact of their being two does not constitute a distinction between them; the determined difference in itself is the principal point.
Fourthly: “The determinateness and the variation thereby established is, however, an inward implicit principle; it is a multiplicity of modification, of relations to surrounding existences, but a multiplicity which remains locked up in simplicity. Determinateness and variation such as this, which remains and goes on in the existence itself, is a perception;” and therefore Leibnitz says all monads perceive or represent (for we may translate perceptio by representation [Vorstellung]). In other words, they are in themselves universal, for universality is just simplicity in multiplicity, and therefore a simplicity which is at the same time change and motion of multiplicity. This is a very important determination; in substance itself there is negativity, determinateness, without its simplicity and its implicitude being given up. Further, in it there is this idealism, that the simple is something in itself distinguished, and in spite of its variation, that it yet remains one, and continues in its simplicity. An instance of this is found in “I,” my spirit. I have many conceptions, a wealth of thought is in me, and yet I remain one, notwithstanding this variety of state. This identity may be found in the fact that what is different is at the same time abrogated, and is determined as one; the monads are therefore distinguished by modifications in themselves, but not by external determinations. These determinations contained in the monads exist in them in ideal fashion; this ideality in the monad is in itself a whole, so that these differences are only representations and ideas. This absolute difference what is termed the Notion; what falls asunder in the mere representation is held together. This is what possesses interest in Leibnitz's philosophy. Such ideality in the same way pertains to the material, which is also a multiplicity of monads; therefore the system of Leibnitz is an intellectual system, in accordance with which all that is material has powers of representation and perception. As thus representing, the monad, says Leibnitz, possesses activity; for activity is to be different, and yet to be one, and this is the only true difference. The monad not only represents, it also changes; but in doing so, it yet remains in itself absolutely what it is. This variation is based on activity. “The activity of the inner principle, by means of which it passes from one perception to another, is desire (appetitus).” Variation in representation is desire, and that constitutes the spontaneity of the monad; all is now complete in itself, and the category of influence falls away. Indeed, this intellectuality of all things is a great thought on the part of Leibnitz: “All multiplicity is included in unity;”(10) determination is not a difference in respect of something else, but reflected into itself, and maintaining itself. This is one aspect of things, but the matter is not therein complete; it is equally the case that it is different in respect of other things.
