In the "Monad°logy" and the writings con nected with it the consequences of the system are developed with particular reference to their bearing upon the nature and destiny of the soul. i Every monad, or simple substance, is an incor poreal unit possessing a capacity of adaptation to the simultaneous condition of other monads which is called perception, and a tendency to spontaneous internal change of state, which is called appetition. An organism is a cluster of such monads, the states of which are more readily seen to be adapted to one another than they are to those of any other monads. In Leibnitz's language, each member of the clus ter ((perceives" or "represents" the rest more dearly and with less confusion than it does any other monads. In each such cluster there is a dominant monad, analogous to a soul, which "represents" the rest with special clear ness, and in which it is, in consequence, particu larly easy to discover the reasons for the suc cession of states in the others. In virtue of the finitude of every monad, however, there is in every monad some element of confused perception, or °passivity," i.e., there are some changes of state in every monad the reasons for which cannot be discovered by us in the monad itself, but have to be found by con sidering other monads. Hence Leibnitz infers that no soul or dominant monad is ever com pletely devoid of a body or system of asso ciated inferior monads. This element of limi tation or confused perception is called "meta physical evil," and is treated as the source both of suffering, or physical, and of wrong doing or moral, evil.
The system of monads now falls into a hierarchy of three grades, according to the degree of the monad's clearness of perception. The perception and appetition of the monad are not necessarily conscious. The condition of the monads which compose the so-called in animate world is similar to that of the soul in profound sleep or swoon; their perceptions are, as we now say, permanently "below the threshold" of distinct consciousness. Leibnitz is thus the author of the doctrine since so fateful in Psychology, of indefinitely minute "unconscious" mental modifications. In ani mals, when fully developed, the formation of sense-organs, which are essentially an apparatus for the collection of stimuli, renders possible the summation of such "minute perceptions" to form a single intensified and therefore con scious perception, and thus provides a basis for memory and association. Those nant monads" which have thus been furnished with heightened perceptions form the second stage in the hierarchy, that of animal souls. In accord with the biological ideas of his time, Leibnitz regards the process of conception and birth essentially as one of development in size; not only is there no generation of life from lifeless matter, but each living germ, even before conception, already contains, on a microscopic scale, the whole organization of the future animal. Death is the converse proc ess of reduction, by which the organism re turns to a microscopic condition; hence, strictly speaking, not only all souls, or dominant monads of organisms, but all organisms them selves, are indestructible. Rational minds form the third and 'highest grade in the system of monads. Their distinguishing characteristic is that, in addition to memory and the power of forming associations of ideas, they possess the power of discerning rational connections be tween truths, and thus of rising to reflective consciousness of the self as a simple substance and of God, the infinite substance. Since the perceptions of every monad are internal states of itself, Leibnitz necessarily agrees with Des cartes in regarding the fundamental notions of rational science as innate, but is more careful than Descartes to give prominence to the neces sity for analysis and reflection before these in nate ideas can be brought to light. Before the reflective process has disengaged them the fun damental categories of rational thought are present in the mind as veins, which have not yet been laid bare, are present in the unshaped block of marble. In virtue of their capacity for rational thought, spirits may be said to be living mirrors not only of the universe, like all other monads, but of God, its cause. God is related
to them not merely as to all created things, after the fashion of an inventor to his machine, but as a prince to his subjects or a father to his children. Thus they form, within the uni verse, a more special society or "city of God" over which God presides not merely in accord ance with natural but also in accordance with moral law. The universe thus presents, within the Pre-established Harmony itself, a second harmony between the physical and moral orders in consequence of which the mechanical se quence of physical causes and effects tends of itself, without any need for special divine in terposition, to an ultimate adjustment of the happiness of each spirit to its moral deserts. Leibnitz's mathematical optimism thus culmi nates in ethical optimism at the cost of a rather hazardous identification of goodness in the metaphysical sense of quantity of existence with goodness in a specifically moral sense.
Two points still call for some further re mark: (1) Leibnitz's conception of matter; (2) his ethical theory.
theory of matter is con ditioned by his discovery of the fundamental defect of Cartesian Mechanics. Descartes had assumed that "quantity of motion," or mo mentum, is independent of the direction of motion, and had consequently been led to base his Mechanics on the assertion that the quan tity of motion in a dynamical system remains constant. Against this view Leibnitz contended that the capacity of a moving body for doing work, its vis viva or "living force," as measured by its effects in moving another body through a greater or a less distance, is proportional not to its momentum, but to its momentum multi plied by its velocity. Hence the correct meas ure of force is not my (momentum) but mu' (vis viva, the double of what we call the Ki netic energy of the body). It is this which is really conserved in dynamical transactions be tween bodies. (The controversies as to the true /measure of force" to which these ob servations gave rise have long been admitted to be largely verbal. If we measure a con stant force by the momentum it generates in a given time, Descartes' formula is, of course, correct; if we measure it by the distance through which it propels a body, that of Leib nitz has to be adopted. Similarly, both the Conservation of Momentum and the Conser vation of Vis Viva, if properly formulated, are truths). The importance of • the point in Leibnitz's philosophy is due to the fact that it led him to regard force as a real entity, different from actual motion, and thus to hold, as against the Cartesians, that the phenomenal physical world cannot be analyzed into exten sion and motion alone. For Leibnitz's own theory of phenomenal matter it is further im portant to distinguish primary from secondary matter. Primary matter itself is something more than mere extension or occupation of space. It is extension plus resistance (a con ception which seems to combine in one inertia and impenetrability). But for this quality of resistance any one state of the physical world would be indistinguishable from any other. Primary matter is however purely passive, and is never to be found existing alone. All physi cal existents contain in addition force, in the sense of spontaneous tendency to change of state. It is the secondary matter thus consti tuted by extension plus resistance plus force i which is studied by the physicist, and held by Leibnitz to be infinitely divided and organized. Secondary matter thus stands, so to say, half way between the monads and the pure abstrac tion of primary matter, and is the matter of which Leibnitz speaks as a "truthful phenome non.' There is really no very, close connection between these dynamical theories and the meta physical system of Leibnitz, though the two are made to look more intimately related than they actually are by a confusion between force in the dynamical sense of rate of change of mo mentum and force in the metaphysical sense of the inherent tendency of the monad to change of state.