Hans Kelsen begins his “Prologue” to the Pure Theory of Law affirming that he sought to purge the law of extralegal interference from the natural sciences and all political ideology. From that consideration, the later critical canon insisted mainly on the second aspect of purification. It is true that the pretension to position the analysis of legal science outside of any ideology is extremely problematic (although, as we shall see, the objective was more complex than it is often stated). But we consider that, without attending to the purification related to the natural sciences, the meaning and the operation that is being carried out in the Kelsenian theory can not be fully understood. Hence, here we will analyze how this work, one of the maximum representations of iuspositivism, with the aim of delineating its own territoriality, maintained a tense dialogue with the implications of ancient and modern philosophies and sciences of nature.