The article introduces Prior’s paper Definitions, Rules and Axioms which deals with Leśniewski’s creative definitions. It presents the origins of Prior’s paper and the discussion which is linked with its final form. Prior’s aim in this paper was to present the Leśniewskian definitions in comparison with Russell’s concept of definitions, demonstrating their advantages and disadvantages. The main source of Prior’s knowledge about the Leśniewskian definitions were Sobociński’s papers and letters, which are stored in the Bodleian library. Although the paper Definitions, Rules and Axioms is a unique attempt at approximating creative definitions, it contains several mistakes. Lejewski identified them in his letter to Prior and also described how they arose. Lejewski’s critique was not severe, however, and Prior coped with it in the introductory page of his paper.