In Chapter 4 there is the attempt to build a bridge between such theory and the traditional institutionalist theories of law. These are reviewed and then supplemented through the neo-institutionalism more recently defended by Neil MacCormick and Ota Weinberger. Neo-institutionalism is then said to be the most promising approach to cope with the ontology of law, though some reform in the standard theory is proposed to render more plausible and less circular the definition given of what an "institution" means and is. In particular, constitutive rules or "declarations' cannot kept outside an institutionalist perspective, though they cannot be said to produce directly "institutional facts' or better the scope of action which the "institution" consists of. They are rather "conditions' to be prescribed in understanding and performing a piece of conduct. This is why a definition of "institution" is advanced whereby constitutive rules are integrated with a notion of efficacy and effective performance.