But it was not till the coming of Leibniz (1646) that spiritual pluralism at last received definite and coherent statement, to gether with logical development. Leibniz started from a con ception of mind directly opposed to that which had been given currency by Locke and the English empiricists. The latter re garded the mind as a passive receiver of impressions from ob jects external to it. But Leibniz realized that it is the essence of mind to be active. In his famous work, the Monadology, he elaborated the theory that reality consists of an infinite number of individual forces or agents, psychic in nature, which he termed "monads." These individual minds or spirits exhibited every degree of mental development and complexity, from that of beings even higher than man (the "angels") right down to that of psychic entities of so low an order that Leibniz described their being as a inens momentanea or mere flash of conscious aware ness. In this hierarchy of mind a complete continuity from one level of development to another was postulated.
Leibniz conceived each monad as reflecting within itself the rest of the universe from its own particular "standpoint." The perceptions of each monad were partly conditioned by its par ticular level of development, and constituted the appearance to it of all the other monads. But Leibniz met with great difficulty at this point. He had conceived the monads as absolutely in dependent reals with no ground of connection between them. But how could the appearance of beings other than itself arise in such an isolated entity as a monad, which, being "windowless" (as Leibniz put it), was impervious to external influence? Leibniz tried to solve this difficulty by his doctrine of "Pre-established Harmony," which involved the introduction of the idea of God into his system. According to this doctrine the development of the universe is the working out of a plan conceived by God when he created the monads. Each monad contains the principle of its own development, but the course of that development is so arranged that, at any instant, the unfolding order of phenomena within each monad is an accurate representation of the rest of the universe at that instant. This may be illustrated by the analogy of a number of clocks set going by the watchmaker so as always to keep time with one another, though actually there is no con nection between them.
The spiritual pluralism of Leibniz, though it continued to exert a dominant influence in Germany until the time of Kant 1804), did not find much vogue among English-speaking phi losophers until comparatively recent times. During the last years of the 19th and the early years of the present century, however, it was adopted and developed, in the reaction against crude materialism, by one or two influential thinkers, including Howison in America and the late James Ward.in England. Though his system is founded on that of Leibniz, Ward introduced very considerable modifications, exposing the weaknesses of the orig final theory and endeavouring to eliminate them. He pointed out that pluralism has to explain away three difficulties. Two of
these are concerned with what he calls respectively "the upper limit" and "the lower limit" of pluralism. If we are to make reality at all intelligible we must, with Leibniz, postulate con tinuous development of the monads. But from what did this de velopment originate and how was it set going? We can trace it downwards to ever lower levels, but we cannot find within it the principle and the explanation of its own origin. On the other hand, whither is it tending? A mere plurality of independently developing beings cannot contain any indication of a satisfying explanation which shall harmonize, and give point to, the manifold separate developments. Thirdly, there is the old difficulty, en countered by Leibniz, as to the impossibility of interaction be tween the monads if these are really independent individuals. Ward comes to the conclusion that a thoroughgoing pluralism is untenable. It must be supplemented by a principle of unity which, while it does not destroy the conception of the monads as real individuals having a "substantival" and not a merely "ad jectival" existence, at the same time enables us to get rid of the difficulties raised by a pure pluralism. Ward finds his unifying principle in a theistic theory, conceiving God as a supreme being transcending the world of the many and yet immanent in it. On the one hand God, as creator, originates monadic develop ment; on the other, He stands as the supreme unity to a har monious co-existence with whom that development is for ever more nearly approaching. Moreover, as immanent in the world, He mediates the interaction between the monads.
It is impossible to enlarge here upon the metaphysical theory thus outlined. It must suffice to point out that it has many im portant possibilities and merits serious consideration. Moreover, it is a healthy corrective to the cruder forms of materialism, for while the latter are ultimately based on conceptions which can be shown to represent pure abstractions, spiritual pluralism starts from a fact of which each of us is certain from his own immediate experience, namely, the concrete existence of such a thing as an individual "mind," or better, perhaps, "spirit." A form of spiritual pluralism is being developed in America by the Personal Idealists; while in England Wildon Carr has pro pounded an interesting monadism with especial reference to the philosophical importance of Einstein's principle of relativity and its apparent implication of the reality and profound significance of the existence of individual observing minds.
BiBuoGRAPHY.—Descartes, Meditations (164o) ; Leibniz, Monad ology (i714) ; Spencer, Synthetic Philosophy: First Principles (1862, 189o) ; Latta, Leibniz: The Monadology, etc. (1898) ; Howison, The Limits of Evolution (1901); James, A Pluralistic Universe (1908) ; Ward, The Realm of Ends (1912) ; C. A. Richardson, Spiritual Plural ism and Recent Philosophy (1919) ; Bertrand Russell, The Analysis of Mind (1921) and The Analysis of Matter (1927) ; Wildon Carr, A Theory of Monads (1922). (C. A. R.)