In morality, a person who is accused of committing a wrong may be able to offeran answer to the allegation made against him. Answers to allegations of wrongdoingcan have a bearing on one's moral responsibility. Analogous remarks can bemade about the law of torts. A defendant whose conduct falls within the definitionof a tort may be able to offer a defence. If accepted, the defence will have abearing on his responsibility within the law of torts. Defences are the subject ofthis book. Its concern is not, for the most part, to identify the rationales for specificdefences (or lack thereof). Neither is it to determine when individual defencesare enlivened (or should be enlivened). These are matters of significant importanceabout which a great deal could be usefully said. But they are not the agendaof this book. Rather, the overarching aims of this book are to explain how defencesoperate as a system and to learn how they are woven into the tapestry that is tortlaw. Although defences are an important part of tort law (there are numerousdefences available to every tort) and have existed since its inception, an analysis ofthis kind- a systematic study of the law of defences as an independent field- hasnever before been undertaken. This introductory chapter lays the foundations ofthe argument that follows.