§7. It doesn’t make sense to suppose that a monad might be altered or re-arranged internally by any other created thing. Within a monad there’s nothing to re-arrange, and there is no conceivable internal motion in it that could be started, steered, sped up, or slowed down, as can happen in a composite thing that has parts that can change in relation to one another. Monads have no windows through which anything could come in or go out! And ·anyway, quite apart from the imperviousness of monads to them, these supposed migrating accidents are philosophical rubbish·: accidents can’t detach themselves and stroll about outside of substances! . . . . So neither substance nor accident can come into a monad from outside.
§10. I take it for granted that every created thing can change, and thus that created monads can change. I hold in fact that every monad changes continually.
(Anish Kapoor. Cloud Gate, 2004.)
§13. This detailed nature must bring a •multiplicity within the •unity of the simple substance. ·The latter’s detailed nature is a ‘multiplicity’ in the sense that it has many components that don’t stand or fall together·. That is because every natural change happens by degrees, gradually, meaning that something changes while something else stays the same. So although there are no •parts in a simple substance, there must be a plurality of •states and of relationships.
† G.W. Leibniz. 1714. Monadology.