made England unfriendly to him. However, the Princess of Wales, as he told all his
correspondents, sided with him against Newton. In spite of her favour, he died neglected.
Leibniz's popular philosophy may be found in the Monadology and the Principles of Nature and
of Grace, one of which (it is uncertain which) he wrote for Prince Eugene of Savoy,
Marlborough's colleague. The basis of his theological optimism is set forth in the Théodicée,
which he wrote for Queen Charlotte of Prussia. I shall begin with the philosophy expounded in
these writings, and then proceed to his more solid work which he left unpublished.
Like Descartes and Spinoza, Leibniz based his philosophy on the notion of substance, but he
differed radically from them as regards the relation of mind and matter, and as regards the number
of substances. Descartes allowed three substances, God and mind and matter; Spinoza admitted
God alone. For Descartes, extension is the essence of matter; for Spinoza, both extension and
thought are attributes of God. Leibniz held that extension cannot be an attribute of a substance.
His reason was that extension involves plurality, and can therefore only belong to an aggregate of
substances; each single substance must be unextended. He believed, consequently, in an infinite
number of substances, which he called "monads." Each of these would have some of the
properties of a physical point, but only when viewed abstractly; in fact, each monad is a soul. This
follows naturally from the rejection of extension as an attribute of substance; the only remaining
possible essential attribute seemed to be thought. Thus Leibniz was led to deny the reality of
matter, and to substitute an infinite family of souls.
The doctrine that substances cannot interact, which had been developed by Descartes's followers,
was retained by Leibniz, and led to curious consequences. No two monads, he held, can ever have
any causal relation to each other; when it seems as if they had, appearances are deceptive.
Monads, as he expressed it, are "windowless." This led to two difficulties: one in dynamics, where
bodies seem to affect each other, especially in impact; the other in relation to perception, which
seems to be an effect of the perceived object upon the percipient. We will ignore the dynamical
difficulty for the present, and consider only the question of perception. Leibniz held that every
monad mirrors the universe, not because the universe affects it, but because God has given it a
nature which spontaneously produces this result. There is a "pre-