David Hume’s theory of law and justice is a central element of his moral philosophy. Hume’s theory of the mind leads to a theory of undesigned social order based on fundamental laws of justice that arise insensibly through experience. The need to secure private property and its free exchange by the performance of promises is the original cause of the emergence of the rules of justice. Hume argues that the moral duty of obedience to authority arises from the need to maintain the rules of justice and that a ruler who violates or fails to uphold justice forfeits the right of allegiance. This paper analyses Hume’s theory and argues that it is epistemologically superior to natural rights theory and provides a powerful justification of property rights and contractual freedom that remains valid today.