Logical monism assumes every proposition has a single subject and single predicate.

Evidence

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 Philosophy, 2114

Categorization

• DEFINE: DEFINE clarifies logical monism and its subject-predicate foundation.
• corroborated: 2 independent passages support this claim — Bertrand Russell, History of Western Philosophy, 2114