How did Gottfried Wilhelm Leibniz’s monads differ from Baruch Spinoza’s idea of one substance?

Gottfried Wilhelm Leibniz’s concept of monads differs fundamentally from Baruch Spinoza’s idea of substance through their opposing metaphysical frameworks: pluralism versus monism.1

• Leibniz’s Pluralism: Leibniz posited that the universe is composed of an infinite number of monads—unique, immaterial, and soullike substances. Each monad acts as an individual center of force that perceives and mirrors the entire universe from its own distinct perspective. Because they lack spatial extension, they are not physical objects but the basic, indestructible building blocks of reality, synchronized by God through preestablished harmony.21

• Spinoza’s Monism: In contrast, Spinoza argued for a strict monism, asserting that there is only one infinite substance, which he identified as God or nature. In this system, what we perceive as individual things—such as living organisms—are not independent substances but merely "finite modes" or expressions of that single, all-encompassing reality.13

While both philosophers were prominent rationalists, Leibniz rejected the idea that reality could be reduced to a single substance, opting instead for a system where the universe is a collection of infinitely many individual, dynamic entities.1