This book explores the various forms of knowledge selection and mediation concerning communication in organizations, particularly focusing on professional communication training courses. The work is based on a corpus study of training catalogues, an interview survey of trainers, as well as ethnographic observations of professional communication training courses. Mediation and Hierarchy of Knowledge on Communication analyzes how the pursuit of certainty contributes to favoring certain types of 'learned' knowledge over others. This analysis reveals that the theoretical frameworks employed in vocational training for communicators predominantly rely on experimental reasoning and explanatory models, drawing upon insights from psychosocial experiments, neuroscience and management science. This quest for certainty has positioned the life sciences as the benchmark for scientific validity, resulting in a form of biologization of communication that this book aims to deconstruct.