This article pretends to provide an insight of the classical conception of Natural Law, as a rule derived from the ontological constitution of man. While the core of the natural normativity (the basic principles of natural law) is known by intuition, its concretions are object of scientific and prudential reasoning. These concretions are not a body of hieratically preset rules, but historical and flexible determinations of general principles that may have components of volitive election. The afore mentioned theory of natural law is, consequently, very different and almost contrary to the rationalist conception of natural law as a set of a priori rules, object of a unique, necessary and algebraic concretion. In addition, this article aims to deepen in the understanding of the juridical dimension of natural law as principle of legal reasoning, main ground of the global law and as a constitutional norm.