What’s in the social world? What matter is it made of? Answering these questions involves building an ontology, i.e. a set of categories for describing and analyzing social phenomena. An ontology is robust if it can deal with a wide range of social situations and is compatible with a broad spectrum of theoretical currents. An ontology for the social sciences must recognize that these sciences study human activities, which implies a distinction between human persons, animals and other types of entities. Entities are engaged in relations and processes, which implies the need for categories along these three dimensions. The use of an emergentist conception allows us to link these three aspects together, and to make the connection between different levels of social reality.