Locke on modes, substances, and demonstration

Locke holds that ideas of modes and relations have special epistemic advantages over ideas of substances. Indeed, he seems to hold that this advantage means that ideas of substances cannot feature in demonstrations. This chapter challenges that view by examining how Locke draws the distinction between ideas of modes, ideas of relations, and ideas of substances and by attempting to explain how this distinction results in ideas of modes and relations having a special epistemic status. Ideas of modes and relations are special because they are real essences, and as a result any consistently conceivable idea of a mode or relation has a possible object. This is not the case for substance ideas. Nevertheless, this does not imply that ideas of substances cannot feature in demonstrations. Rather, Locke merely thinks that demonstrations involving substances are rare, but not in principle impossible. The explanation of this appeals to the kinds of simple ideas that make up a significant part of our ideas of substances: they are collections of ideas of qualities and powers that are rarely related in the ways required for demonstrative knowledge.