This paper examines the validity of naturalistic arguments in the discussion on the legitimacy of biotechnological measures. This objective is approached through critical evaluation of the concept of naturalness used in that context. In general, the concept is based on the Aristotelian separation of art and nature and thus meets first criticism. This is followed by an in-depth examination of the validity of naturalistic arguments being used as legitimation for legal standards of conduct in the field of biomedicine. Ultimately, only the arguments providing the better reasons for a specific position are able to convince in legal discussions. Naturalistic arguments prove to be obstructive in the debate on new biotechnological procedures.