In law as well as in ordinary life, it matters what rules societies have; but what does it mean for a society to have a rule? HLA Hart’s famous answer is that for a society to have a rule is for there to be a certain social practice in that society, consisting of an external, behavioural aspect and an internal, attitudinal aspect. Hart’s ‘practice theory’ dominates thinking in jurisprudence about social rules, but, I argue, there are serious problems with it. It would be better to adopt what I call the ‘acceptance theory’. In the early chapters of this thesis, I argue that the practice theory is both overinclusive and underinclusive. It is overinclusive because Hart’s description of the ‘internal aspect’ is too general. It is underinclusive because the ‘external aspect’ is unnecessary. Once these criticisms are taken into account, what remains of the practice theory is the idea that a society has a rule because its members have a certain attitude. I spend much of this thesis determining the features of this attitude. Ultimately, I focus on the attitude known as “acceptance” in the philosophy of action. Acceptance of a proposition simulates belief in that proposition, though it may be held independently of that belief. I argue that a person or society has a rule when that person, or that society’s members, accept that some action ought to be performed, whatever their beliefs about the matter. This theory incorporates the plausible core of the practice theory, while avoiding its problems.