During the beginning of the cinema media era around the turn of 20th century, a debate prevailed in Sweden about the film’s harmful and negative moral impact on children and young people. This study aims to investigate the debaters’ knowledge claims regarding a preview of a publicly shown film and if it is related to their morality. The following questions were asked: 1. What knowledge claims are made in connection with the need for prior control over the film that is shown in public? 2. What is presented as the basis for the legitimacy of these knowledge claims? 3. How do these knowledge claims change during its path to legal text? 4. What was the relationship between the presented knowledge claims and a moral perception prevailing among the debaters? A debate paper, a parliamentary motion from 1909 and the public inquiry before the decision to create the Swedish Agency of Movie Censors were analysed with argumentation analysis and thematic analysis with a perspective of history of knowledge. This study points at various aspects such as medical, psychological, social structure and moral arguments, which were put forward as arguments for a regulation of the new film medium. The knowledge claims were justified by professional expertise, general knowledge, experiences from other countries and the debaters’ prior knowledge of the subject. A strong fear of ruining a good existing moral conception that exists in the growing generation emerges. The analysis describes a change in knowledge claims: the moral arguments diminish along the way towards the legal text. Furthermore, the study points at the necessity for continued research to increase the understanding of how legitimacy of knowledge is created by an influential group and then passed on.