This paper re-reviews Vickery’s book On retrieval system theory, first published 50 years ago, and discusses the changing nature of theoretical work on information retrieval and the possibility of developing a general theory of IR. Stephen Robertson writes: ‘What kinds of theory or theories do we need for the field of information retrieval?’ Brian Vickery’s book whose title I have purloined was first published in 1961; in the preface he makes the following disclaimer: ‘There is as yet no unified theory of retrieval systems’. I have made the same statement many times myself, and it is as true now as it was a half-century ago. The number of papers published in the field of information retrieval, in every year of the first decade of the third millennium, would astonish the Brian Vickery of 1961, and many of these papers appeal to theoretical arguments of various more-or-less formal kinds. But can we expect, and do we need or want, a unified theory? In this talk I will attempt some discussion of these issues.