It has often been claimed that Locke’s agnostic remarks in the Essay represent his suspension of philosophical judgment on crucial ontological issues or his hesitation over which metaphysical stance to adopt. Against this often-raised criticism, I argue that Locke actually held a clear position—a type of functionalism about thingness in general, whether macro or micro, or whether mental or physical. What Locke refers to as a ‘nominal essence’, I further argue, represents a set of functional roles that a thing plays (...) in order to be classified as of a kind to which it belongs. Our empirical knowledge about things—confined to their nominal essences—can only tell us about their functional roles but not their intrinsic properties that realize those roles. One remains therefore incurably ignorant about the intrinsic property of things in themselves. I explore the historical and philosophical significance of Locke’s functional approach. (shrink)