The four-principle approach to biomedical ethics is used worldwide by practitioners and researchers alike but it is rather unclear what exactly people do when they apply this approach. Ranking, specification, and balancing vary greatly among different people regarding a particular case. Thus, a sound and coherent applicability of principlism seems somewhat mysterious. What are principlists doing? The article examines the methodological strengths and weaknesses of the applicability of this approach. The most important result is that a sound and comprehensible application (...) of the four principles is additionally ensured by making use of the organizing meta-principle of common morality, which is the starting point and constraining framework of moral reasoning. (shrink)