Abstract The common law tradition has always been sceptical of philosophy, prioritising the importance of practical experience in real world cases over abstract general ideas. This article attempts to demonstrate how the ideas of two philosophers—Fuller and Wittgenstein—help to explain the widespread use of the notion of purpose in real world case law. Fuller’s conception of law as the enterprise of subjecting human conduct to the governance of rules illuminates the common law’s respect for the agency of legal subjects, whether that be respecting parties’ purposes in making a contractual bargain or using purpose as a basis for limiting liability in tort. Similarly, Wittgenstein’s conception of language demonstrates the necessity of resort to purpose in interpretation, as the open-textured nature of words means that legal rules only become determinate when used in specific contexts for specific purposes. The article focuses primarily on the United Kingdom’s (UK) legal system, with its two tracks of the common law and parliamentary legislation, but also extends the argument to examples drawn from forms of supranational law to demonstrate that the analysis is capable of being generalised beyond the UK’s legal system.