This paper tests the claim, made by H.L.A. Hart, that nothing is a legal system that fails to include certain obligations familiar in morality . This ‘minimum content’ thesis was rejected by Hans Kelsen, among others. Hart’s arguments for the minimum content thesis are unsound; but the thesis is correct and a different defense of it is offered. Two general worries about the thesis are then addressed. Brian Leiter argues that, since law is an artefact, it has no essential properties at all and, a fortiori, no essential content. This is shown to involve several errors. Others argue that Hart’s view about the relation between law and morality is not a theoretical thesis at all: it is the practical thesis that we should try to improve law by improving the concept of law. It is shown that this is not a possible interpretation of any of Hart’s arguments: ‘normative positivism’ has no Hartian foundations.