According to Leibniz, the monads or ultimate elements of existence are constituted by the two essential properties of psychic life, perception and appetency. In this connection two points have to be made clear. What he calls bare monads--_i.e._, the components of what is known as inorganic matter--although percipient, are not conscious of their perceptions; in his language they do not _apperceive_. And he endeavours to prove that such a mentality is possible by a reference to our own experience. We hear the roaring of waves on the seashore, but we do not hear the sound made by the falling of each particle of water. And yet we certainly must perceive it in some way or other, since the total volume of sound is made up of those inaudible impacts. He overlooks the conceivable alternative that the immediate antecedent of our auditory sensations is a cerebral disturbance, and that this must attain a certain volume in order to produce an effect on our consciousness. The other point is that the appetency of a monad does not mean an active impulse, but a search for more and more perceptions, a continuous widening of its cognitive range. In short, each monad is a little Leibniz for ever increasing the sum of its knowledge. At no stage does that knowledge come from experience. The monad has no windows, no communication of any kind with the external world. But each reflects the whole universe, knowing what it knows by {63} mere introspection. And each reflects all the others at a different angle, the angles varying from one another by infinitesimal degrees, so that in their totality they form a continuous series of differentiated individuals. And the same law of infinitesimal differentiation is observed by the series of progressive changes through which the monads are ever passing, so that they keep exact step, the continuity of existence being unbroken in the order of succession as in the order of co-existence. Evidently there is no place for free-will in such a system; and that Leibniz, with his relentless fatalism, should not only admit the eternal punishment of predestined sinners, but even defend it as morally appropriate, obliges us to condemn his theology as utterly irrational or utterly insincere. In this system animal and human souls are conceived as monads of superior rank occupying a central and commanding position among a multitude of inferior monads constituting what we call their bodies, and changing _pari passu_ with them, the correspondence of their respective states being, according to Leibniz, of such a peculiarly intimate character that the phenomena of sensation and volition seem to result from a causal reaction instead of from a mechanical adjustment such as we can imagine to exist between two clocks so constructed and set as to strike the same hour at the same time. This theory of the relations between body and soul is known to philosophy as the system of pre-established harmony.