How to distinguish law from non-legal but systematic and rule-guided practices of legal officials? This issue features prominently in the debate on ‘positive originalism’ in US constitutional law, and in similar fundamental controversies in other legal orders. I take it as a question about content and constitution of ultimate rules of recognition. Legal philosophers have been too quick in dealing with this problem. I argue that there is more space to claim that non-officials have a constitutive relationship with the content (...) of the law, thus potentially providing a standard to distinguish legal and non-legal practices of officials. However, to the extent officials play a constitutive role in the law, what matters is their genuine acceptance of ultimate rules of recognition. To show this, I develop the concept of acceptance of a social rule by specifying the requirement of genuineness of acceptance and the role of mental dispositions associated with acceptance. (shrink)