"For since it is impossible for a created monad to have a physical influence on the inner nature of another, this is the only way in which one can be dependent on another"

The quote suggests that a developed monad (a basic system of truth in Leibniz's viewpoint) can not directly impact another monad's physical nature. For that reason, the only method which one monad can be based on another is through an esoteric connection or influence. Leibniz believed that all monads were interconnected which the whole universe was a harmonious, interconnected system.