In ontology the underlying nature of things is sought. Thus it is that the roles things play can sometimes be neglected. Some different sorts of things that get called roles: places in relations, ways of participating in activities, and ways of being a part of a system, are considered. Roles are then analysed from a 4-dimensional and extensional perspective. We find that in individuals a role is a state of some individual that participates in a relationship or an activity, whereas with classes it is the class itself that participate in a place in a relation. In systems (both social and functional roles) we find that roles are replaceable parts of the system that are the purpose the part plays in the whole. A 4-dimensionalist ontology of roles, linked to an upper ontology is presented, and this is used to set out the key properties of roles as found here and by others.
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