Scientific explanation can warrant commitment without establishing certainty. Defeasible ontology is selective: it distinguishes the entities, relations, capacities, constraints, and forms of organisation that explanatory practices have reason to preserve from the representational features that remain provisional. It is revisable because later evidence may change either the commitment or its interpretation. Revisability does not reduce ontology to convenience. It expresses the fact that scientific claims about reality are warranted through practices that can succeed, conflict, and fail.