The present thesis is primarily an exercise in philosophy of law. However, it can also be viewed as a contribution to the recurring philosophical investigations about the nature of prescriptive rules (and the directive or prescritive use of language in general), encompassing: an inquiry about rules and their characteristic and contigent marks, and a research of several ways by means of which they can play a part in the practical reasoning of its addressees. In order not to use language on holiday, the discussion about rules will be held on the stage set up by conceptual positivism. The aim is to point out some of the main problems that must be faced when rules are taken seriously. In a nutshell, the thesis is an incursion into the philosophical sources of hard cases, when law is conceived as a system of rules.