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.

Bertrand Russell, History of Western Philosop…, loc. 2128