The pre-established harmony

Leibniz has painted himself into a corner — deliberately. Monads are windowless: no substance causally acts on any other. Yet the world is a symphony of correspondence. I will to lift my arm, and up it goes. I stub my toe, and pain follows. The states of my mind and the states of my body track each other with flawless precision. If there is no causal link between them, this coordination is a miracle demanding explanation.

Leibniz’s answer is the pre-established harmony (harmonie préétablie). At the moment of creation, God so arranged — “programmed” — the inner nature of every monad that its sequence of internal states unfolds in exact mutual correspondence with the states of all the others. Each windowless substance spins out its own melody from within, yet all the melodies were composed to form one perfect chord (Monadology §51–56, 78–81).

So when I will to raise my arm and the arm rises, there is no push from mind to body or body to mind. My soul, following its own internal appetition, arrives at the volition; my body, following the laws of motion inscribed in its monads, arrives at the rising — and the two were synchronized in advance to meet at that instant. Apparent causation between substances is really the harmonious co-unfolding of independent, windowless clocks.

Notice the boldness: on this view, even if your body vanished, your soul would keep generating exactly the same sequence of perceptions — including the perception of a body. The correspondence lives entirely in the way the melodies were written, not in any real contact. Every monad is, in this sense, a self-contained world that happens to agree with all the others by design.

This coordinating principle rests on a deeper law Leibniz derives from the Principle of Sufficient Reason: the Identity of Indiscernibles (principium identitatis indiscernibilium). There cannot be two things exactly alike — two substances sharing all their properties. If they were qualitatively identical in every respect, there would be no sufficient reason for God to treat them as two rather than one; so they would be one (Monadology §9; correspondence with Clarke).

For the monads this is essential. Every monad mirrors the whole universe from its own unique point of view, and no two share a viewpoint — so no two have the very same perceptions, and each is qualitatively distinct from every other (Monadology §57–60). The Identity of Indiscernibles guarantees that the infinite multitude of monads is a multitude of genuine individuals, each contributing something no other does. That variety of viewpoints is exactly what the pre-established harmony orchestrates into one world.