A PHILOSOPHY OF INDIVIDUALS.--Leibniz's criticism of Descartes and the mechanical school proceeded along different lines from that of Spinoza, who, as we have seen, accepted the mechanical view as the basis of his speculation. An axiom of that view was (as we know) the conservation of motion. For this conservation of _motion_, Leibniz substitutes the conservation of _force_ as being logically the more fundamental concept. True reality, according to him, is not _motion_ itself, but the _force which is its cause_. Force and existence became for him identical terms; to work and to exist were the same. That force is the true reality, Leibniz expressed in the language of his time by saying, "Force is substance, and all substance is force"--a proposition which would not be repudiated by modern science--and upon this statement his philosophy is built. But it was not "force in general" or some "universal force" that was regarded by him as the final reality: Leibniz was not a forerunner of Herbert Spencer. Reality for him consisted in _individual centres of force_--a multitude of individual and independent beings, each with its own idiosyncrasy, and following its own lines. Existence was, in fact, for him, _individual_. It was the _individual_ centres of force--not _general_ principles, universal substances, laws or forces--that make up reality. DOCTRINE OF MONADS.--This view of reality was formulated by Leibniz in his famous doctrine of "monads." "Monad" was the technical name applied by him to those absolute individuals which he regarded as constituting true reality. The word, meaning "unity," was simple and appropriate. And he declared that the "monad," to be rightly understood, must be regarded as analogous to our own souls. This principle of analogy was described by Leibniz as _mon grand principe des choses naturelles_. Thus reality was interpreted by him not in physical but in psychical terms, or if the expression be preferred, in terms of personality.[10] Of these "monads" there exist, according to this view, infinitely many degrees. In fact all existence differs only in degree from our own. Even between mind and matter there is only a quantitative and not a qualitative gulf. For there are sleeping, dreaming, and more or less waking monads; and matter is a form of unconscious mind; the monads which compose material objects being "minds without memory," "momentary minds." Let Leibniz speak for himself:-- "Each portion of matter is not only infinitely divisible, but is also actually subdivided without end.... Whence it appears that in the smallest particle of matter there is a world of creatures, living beings, animals, entelechies, souls. Each portion of matter may be conceived as like a garden full of plants, or like a pond full of fishes.... Thus there is nothing fallow, nothing sterile, nothing dead in the universe...."[11]