Behavior, language, development, identity, and science-all of these phenomena are commonly characterized as ‘social’ in nature. But what does it mean to be ‘social’? Is there any intrinsic ‘mark’ of the social shared by these phenomena? In the first book to shed light on this foundational question, twelve distinguished philosophers and social scientists from several disciplines debate the mark of the social. Their varied answers will be of interest to sociologists, anthropologists, philosophers, psychologists, and anyone interested in the theoretical foundations of the social sciences.