Abstract

The world is occupied by many and varied things. What constitutes their thingness? In the Essay, Locke addresses this question in Book II, Chapter xxiii, titled ‘Of our Complex Ideas of Substance’, wherein the much-contested definition of ‘substratum’ appears—‘a supposed but unknown support of the Qualities’. Most significant in this definition are the dual qualifiers that Locke uses: ‘supposed’ and ‘unknown’. This paper examines this two-qualifier definition, illuminating the historical and philosophical significance it may have. There have been two rival readings. The first takes Locke’s substratum to be a bare substratum; and the second identifies it with what Locke terms as ‘real essence’—i.e. ‘a real Constitution of the insensible Parts’. Critically reviewing these two major interpretations, I attribute to Locke a type of functionalism, according to which the status of a substratum is determined by its functional role of ‘uniting’ a bundle of qualities into an individual substance.