Possibilities haunt history. The force of our explanations of events turns on the alternative possibilities these explanations suggest. It is these possible worlds which give us our understanding; and in human affairs we decide them by practical rather than theoretical judgement. In his widely acclaimed account of the role of counterfactuals in explanation, Geoffrey Hawthorn deploys extended examples from history and modern times to defend his argument. His conclusions cast doubt on existing assumptions about the nature and place of theory, and indeed of the possibility of knowledge itself, in the human sciences.