Matthew Duncombe’s chapter ‘Relative Concepts’ asks: what are relative concepts according to ancient philosophers? Duncombe argues that Plato, Aristotle, and the Stoics have a clear concept of relatives, distinct from incompleteness approaches, which he calls ‘constitutive relativity’. The core idea of constitutive relativity is that a relative is constituted precisely by the relation it bears to an exclusive correlative. Duncombe discusses particular philosophers and schools in detail. The examination of Agathon’s speech in Plato’s Symposium illustrates that Socrates understands relative concepts in general and love in particular, on the constitutive model. Aristotle’s concept of relatives in Categories 7 draws on Plato, but Aristotle addresses a worry that relative concepts might be vacuous. Duncombe argues that a Stoic relative concept is the concept of a relative that relates exclusively to a correlative. He examines Sextus’ sceptical argument, which raises a worry about any conception of relativity where relatives relate exclusively to their correlatives.