This third volume of Principia iuris, dedicated to the theory of law, draws the logical syntax of theoretical discourse and the complexity of the legal structures that forms the object. According to the axiomatic method adopted, all terms and all the theses of the theory, except for a limited number of primitive terms and thesis, are each defined by other theoretical terms and the other derived from other theses of the theory on the basis of rules of formation and transformation of previously established. The price is the formalization of the use, next to the common language, a complex symbolic language, with its outfit of formulas and calculations. Its benefits are sure the rigorous reconstruction of the entire legal language theory, the maximum simplification of the concepts compatible with their clarity and explanatory power, the internal consistency of the speech without a lot of aporia and ambiguity of ordinary language, exposure to logical control of all the claims proved and the assessment and non-critical logic of all argument taken in the form of postulates or definitions.