I shall speak for the Pure Mathematics Research Seminar at the University of East Anglia in Norwich on Monday, 25 February, 2019.

**Abstract.** An old argument, heard perhaps at a good math tea, proceeds: “there must be some real numbers that we can neither describe nor define, since there are uncountably many real numbers, but only countably many definitions.” Does it withstand scrutiny? In this talk, I will discuss the phenomenon of *pointwise definable* structures in mathematics, structures in which every object has a property that only it exhibits. A mathematical structure is *Leibnizian*, in contrast, if any pair of distinct objects in it exhibit different properties. Is there a Leibnizian structure with no definable elements? Must indiscernible elements in a mathematical structure be automorphic images of one another? We shall discuss many elementary yet interesting examples, eventually working up to the proof that every countable model of set theory has a pointwise definable extension, in which every mathematical object is definable.

Lecture notes – Must every number be definable? Norwich Feb 2019