Full Idea
A monad's natural changes come from an internal principle, ...but there must be diversity in that which changes, which produces the specification and variety of substances.
Gist of Idea
Changes in a monad come from an internal principle, and the diversity within its substance
Source
Gottfried Leibniz (Monadology [1716], §11-12)
Book Reference
Leibniz,Gottfried: 'Philosophical Essays', ed/tr. Arlew,R /Garber,D [Hackett 1989], p.214
A Reaction
You don't have to like monads to like this generalisation (and Perkins says Leibniz had a genius for generalisations). Metaphysics must give an account of change. Succeeding time-slices etc explain nothing. Principle and substance must meet.