The emergence of possible-worlds semantics for modal logic in the late 1950s and early 1960s led to a recognition that the structure of modal and temporal logic can be treated in an exactly parallel way, with ‘possible worlds’ in the one case playing the same role as moments of time in the other. While this has been known now for many years there has been considerable reluctance among philosophers to ask why it should be so, and to embrace its consequences. Most of those who have written on the topic have had the aim of attempting to explain why the formal parallel has little philosophical significance. The present volume is, we believe, the first book-length work to address the phenomenon explicitly and present the case for its power.

The work has been supported by a grant from the New Zealand Government's Marsden Fund, administered by the Royal Society of New Zealand. We would express our thanks to the Marsden Fund, and to our three universities, Massey University, the University of Auckland and the Victoria University of Wellington, for providing the resources for the research to be undertaken. Some of the material was also used in a graduate course at Texas A&M University in the (northern) spring semester of 2007, and we are grateful to A&M for that opportunity. We would like to thank the readers for Cambridge University Press, whose helpful and perceptive comments enabled us to reorganise the chapters of the book so that the structure of the argument becomes more easily apparent.