In 1978, Neil MacCormick published the book Legal Reasoning and Legal Theory with the goal of constructing a descriptive and normative theory of legal reasoning, compatible with the legal positivism of H. L. A. Hart. The author used the rational reconstruction of concrete pre-selected cases as the method to present his theory. The analysis of judicial decisions offers empirical backing to MacCormick s theses and highlights the focus of his theory of legal reasoning: the study of the process of legal justification. Based on this focus, the author establishes the split between first-order justification and second-order justification. This dissertation only analyzes the first- order justification, which is related with the formal correction of legal reasoning. The model adopted to evaluate this characteristic is deduction, especially the mixed hypothetical syllogism of traditional logicians. According to the Scottish author, the judicial decision that attempts to subsume the variables of the concrete case in the universal components of the operative fact of the legal rule, coming to the conclusion of a normative consequence of that same rule, will be justified in a State that promotes the Rule of Law.