We all know that being reasonable is important in daily life. Beyond daily life, major political and ethical theorists give central place to reasonableness in their accounts of just and moral behaviour. In the law, at least in the Anglo-American setting, reasonableness is the standard for a wide range of behaviour, from administrative decisions to torts. But what is it to be reasonable? In answer, I provide a genealogical account of reasonableness. The functional perspective afforded by a genealogical account has (...) numerous payoffs, explaining important features of reasonableness, and enabling us to respond to worries about vagueness or subjectivity in the meaning of ‘reasonable’. (shrink)