The present work aims to describe the role of the reasoning of judicial decisions in the Hermeneutic Critique of Law (CHD). As a research problem, it focuses on the explanation about the imperative of reasoning decisions. Justified by the relevance of the theory, the research seeks to centralize an issue addressed in decades of work. The hypothesis: the grounds for decisions is a condition of possibility for the realization of Fundamental Laws. The method used is logical-deductive, based on bibliographical research. In general, we tried to present an overview of the premises, theses and consequences of CHD. Therefore, the research was subdivided into a theoretical analysis of the CHD to later carry out its application in the case of Habeas Corpus no 82.424: The Ellwanger Case. It was concluded that in terms of jurisdiction, any compression must be adequate to the intersubjective agreement called the Constitution. In even more practical terms, it was found that the foundation of judicial decisions is fundamental because it is precisely in its implementation that any authentic Law can be based.