A significant ontological commitment is required to sustain metaphysical realism—the view that there is a single, objective way the world is—in order to defend it from common sense objections. This involves presupposing the existence of properties (or tropes, or universals) and relations between them which define the objective structure of the world. This paper explores the grounds for accepting this ontological assumption and examines a sceptical argument which questions whether, having assumed the world is objectively divided into fundamental properties, we could ever know which properties these are. It then assesses the responses available to the metaphysical realist, arguing that the sceptical difficulty cannot merely be dismissed by means of another assumption in the manner of radical scepticism, as David Lewis suggests, but that the sceptic's argument might be defused by the non-question-begging success of some form of strong scientific realism which links the predicates of our scientific theories directly to the fundamental properties the world contains. It remains unclear however whether this widely accepted metaphysical theory can find principled philosophical support.