The Pure Theory of Law emerged in the first half of the 20th century as a vigorous criticism to the traditional juridical theory. This opposition consists in an impressive epistemological program, whose main target is to eliminate from the study of Law both political elements and specific methods shared with other sciences. Its author, Hans Kelsen, intended to provide a theoretical apparatus to the jurist which would allow him to contemplate his object with the same qualty and accuracy already attained in the natural sciences. In order to do so, he found necessary the juridical knowledge had both scientificity and autonomy, and his famous doctrine was conceived to achieve that. Nevertheless, in spite of the undeniable progress that came with it, the Pure Theory of Law seems to suffer from an insoluble problem that might compromise its integral sustainability. It is the Tacit Alternative Clause, Kelsenian thesis of which we shall treat here and whose delineation is preceded by the investigation of the premises and concepts which are crucial to its full understanding.