Spinoza’s metaphysic is the best example of what may be called ‘logical monism’—the doctrine, namely, that the world as a whole is a single substance, none of whose parts are logically capable of existing alone. The ultimate basis for this view is the belief that every proposition has a single subject and a single predicate, which leads us to the conclusion that relations and plurality must be illusory.

Bertrand Russell, History of Western Philosop…, loc. 2114