"It follows from what we have just said, that the natural changes of monads come from an internal principle, since an external cause would be unable to influence their inner being"

In this quote, Leibniz is specifying that the modifications that happen within monads (basic units of existence according to his ideology) come from within, as opposed to being caused by exterior elements. He argues that an outside cause would not be able to influence these changes as they are intrinsic to the monad's inner being. Essentially, Leibniz is promoting the idea of self-contained presence, where the changes that take place are driven by interior variables instead of being affected by outside pressures.