This book is the second in a series of four that is concerned with defences to liability arising in private law. We felt, and still feel, that the topic has not received the attention that it deserves. 1 We are not alone in holding this view. 2 By contrast, defences have dominated the research agendas of many scholars of the criminal law.3 The asymmetry in attention to defences in these different fields is striking in part because of the apparent parallels between the two domains. For instance, the distinction in private law between causes of action and defences arguably mirrors that between offences and defences in the criminal law.