"La nature de la substance consistant à mon avis dans cette tendance réglée de laquelle les phénomènes naissent par ordre."--_Lettre à M. Bayle_. "Car rien n'a mieux marqué la substance que la puissance d'agir."-- _Réponse aux Objections du P. Lami_. "S'il n'y avait que des esprits, ils seraient sans la liaison nécessaire, sans l'ordre des tems et des lieux."--_Theod_. Sect. 120.] He parts company with Spinoza on the question of individuality. Substance is homogeneous; but substances, or beings, are infinite. Spinoza looked upon the universe and saw in it the undivided background on which the objects of human consciousness are painted as momentary pictures. Leibnitz looked and saw that background, like the background of one of Raphael's Madonnas, instinct with individual life, and swarming with intelligences which look out from every point of space. Leibnitz's universe is composed of Monads, that is, units, individual substances, or entities, having neither extension, parts, nor figure, and, of course, indivisible. These are "the veritable atoms of nature, the elements of things." The Monad is unformed and imperishable; it has no natural end or beginning. It could begin to be only by creation; it can cease to be only by annihilation. It cannot be affected from without or changed in its interior by any other creature. Still, it must have qualities, without which it would not be an entity. And monads must differ one from another, or there would be no changes in our experience; since all that takes place in compound bodies is derived from the simples which compose them. Moreover, the monad, though uninfluenced from without, is changing continually; the change proceeds from an internal principle. Every monad is subject to a multitude of affections and relations, although without parts. This shifting state, which represents multitude in unity, is nothing else than what we call _Perception_, which must be carefully distinguished from _Apperception_, or consciousness. And the action of the internal principle which causes change in the monad, or a passing from one perception to another, is _Appetition_. The desire does not always attain to the perception to which it tends, but it always effects something, and causes a change of perceptions. Leibnitz differs from Locke in maintaining that perception is inexplicable and inconceivable on mechanical principles. It is always the act of a simple substance, never of a compound. And "in simple substances there is nothing but perceptions and their changes." [20] [Footnote 20: _Menadol_. 17.]