Full Idea
The whole, if it is assumed to be body or matter, is nothing other than all of its parts; but this is absurd, since there aren't any smallest parts. Therefore there really exist only minds and their perceptions.
Gist of Idea
A whole is just its parts, but there are no smallest parts, so only minds and perceptions exist
Source
Gottfried Leibniz (Calculus Ratiocinator [1679], A6.4.279), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 7
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.288
A Reaction
Leibniz is sometimes labelled as an 'idealist', but this text is unusual in being so explicit, and he was mainly concerned to explain the reality of individual bodies. Monads were his final attempt to do this, not an attempt to escape into pure minds.