"But in simple substances the influence of one monad over another is ideal only"

In this quote, Gottfried Leibniz is going over the principle of monads, which are indivisible and immaterial entities that exist in all things. He is stating that when one monad affects another monad, it is a perfect or abstract influence rather than a physical one. This recommends that despite the fact that monads might interact with each other, they keep their distinct individuality and can not be physically integrated or separated.