High Quality Content by WIKIPEDIA articles! The closed world assumption (CWA) is the presumption that what is not currently known to be true, is false. The same name also refers to a logical formalization of this assumption by Raymond Reiter. The opposite of the closed world assumption is the open world assumption (OWA), stating that lack of knowledge does not imply falsity. Decisions on CWA vs.OWA determine the understanding of the actual semantics of a conceptual expression with the same notations of concepts. A successful formalization of natural language semantics usually can not avoid an explicit revelation of the implicit logical backgrounds based on whether CWA or OWA.