(_c_) To the Cartesian conception of substances as inert entities endowed only with motions communicated to them _ab extra_, the mechanical or atomist conception of reality, as it is called, Leibniz opposed the other extreme conception of substances as _essentially active entities_. For him substance is an _ens præditum vi agendi_: activity is the fundamental note in the concept of substance. These essentially active entities he conceived as being all _simple_ and _unextended_, the corporeal no less than the spiritual ones. And he gave them the title of _monads_. It is unnecessary for our present purpose to go into any details of his ingenious dynamic theory of the universe as a vast system of these monads. We need only remark that while combating the theory of inert substances he himself erred in the opposite extreme. He conceived every monad as endowed essentially with active tendency or effort which is never without its effect,—an exclusively _immanent_ effect, however, which is the constant result of constant immanent action: for he denied the possibility of transitive activity, _actio transiens_; and he conceived the immanent activity of the monad as being in its nature _perceptive_,(243) that is to say, _cognitive_ or _representative_, in the sense that each monad, though “wrapt up in itself, doorless and windowless,” if we may so describe it, nevertheless mirrors more or less inchoatively, vaguely, or clearly, all other monads, and is thus itself a miniature of the whole universe, a microcosm of the macrocosm. Apart from the fancifulness of his whole system, a fancifulness which is, however, perhaps more apparent than real, his conception of substance is much less objectionable than that of Descartes. For as a matter of fact every individual, actually existing substance is endowed with an internal directive tendency towards some term to be realized or attained by its activities. Every substance has a transcendental relation to the operations which are natural to it, and whereby it tends to realize the purpose of its being. But nevertheless substance should not be defined by action, for all action of created substances is an accident, not a substance; nor even by its transcendental relation to action, for when we conceive it under this aspect we conceive it as an _agent_ or _cause_, not as a _substance_ simply. The latter concept abstracts from action and reveals its object simply as “a reality existing in itself”. When we think of a substance as a principle of action we describe it by the term _nature_.