The focus of this work, the search for the limits of instrumental reason in criminal law, is motivated by two insights: the continuous instrumentalization of preventive criminal law and the unbounded aspect of its legal development. The investigation is based on the premise that the prevailing prevention paradigm follows that of "instrumental" - purpose-driven - reason. It primarily questions the effectiveness of the means employed, without assessing the rationality of the pursued goals in themselves or elevating them to a condition of legitimacy. The work is committed to formulating such legitimizing conditions, delineating the boundaries of a constitutional criminal law. Based on a legal idea of limiting state power, the substantive criteria are the freedom of the individual as a fundamental assumption and the unconditionality of an absolute guarantee of human dignity.