This paper investigates, in a historical and cultural perspective, the meaning of the word reasonable, and in particular, of the phrases reasonable man and reasonable doubt, which play an important role in Anglo-American law. Drawing on studies of the British Enlightenment such as Porter (2000), it traces the modern English concept of ‘reasonableness’ back to the intellectual revolution brought about by the writings of John Locke, who (as Porter says) ‘replaced rationalism with reasonableness, in a manner which became programmatic for the Enlightenment in Britain’. The paper also argues that the meaning of the word reasonable has changed over the last two centuries and that as a result, the meaning of the phrases reasonable man and beyond reasonable doubt has also changed; but since these phrases were continually used for over two centuries and became entrenched in Anglo-American law as well as in ordinary language, and since the older meaning of reasonable is no longer known to most speakers, the change has, generally speaking, gone unnoticed. On a theoretical level, the paper argues that meaning cannot be investigated in a precise and illuminating manner without a coherent semantic framework; and that a suitable framework is provided by the ‘NSM’ semantic theory.