 In textbooks on set theory we also find a notion of structure. Roughly, the set theoretic definition says that a structure is an ordered \(n+1\)-tuple consisting of a set, a number of relations on this set, and a number of distinguished elements of this set. But this cannot be the notion of structure that structuralism in the philosophy of mathematics has in mind. For the set theoretic notion of structure presupposes the concept of set, which, according to structuralism, should itself be explained in structural terms. Or, to put the point differently, a set-theoretical structure is merely a system that instantiates a structure that is ontologically prior to it.