After defining monads as partless and showing there must be such things,* Leibniz argues that they can neither come into existence or pass out of it as natural things do, since in nature things arise when their parts come together and cease existing when they fall a-part, and both are excluded from the monad by definition.
Objection 2: But if a monad cannot either arise or cease to be as natural things do, it seems it can’t preserve itself in existence as they do either. Natural things persevere by hanging together, and monads can’t hang together any more than they might be put together or fall apart. Whatever we say about the boundaries of existence has to be said about its midpoints.
*Objection 1: On the one hand, Leibniz assumes that there must be simple things because there are complex ones, and it’s clear that he means there must be partless things because there are things with parts. He then almost immediately argues that monads cannot be quantities since quantities have no least part. But if quantities can have parts without having a least such, why can’t complex things have a parts without having a monad?