Was John Locke a monist?
Locke was an extreme empiricist, meaning that he believed knowledge can only come from experience and the senses. The monism – dualism issue was prominent among rationalists like Descartes and Spinoza. I believe Locke would say that there is a reality out there which exists independent of our senses, but we can never know what it is except how it is experienced by us. So he would consider your question unanswerable. He would probably say something like “yes there is a substance out there, more likely there are many substances, but we will never be able to know them except as we experience them, and that is not their pure form”. BTW this is exactly Kant’s position. Kant said we can never know “things-in-themselves”, which he called noumena.
