 This conception of states of affairs as complexes makes them useful for semantics. If sentences refer to such states of affairs, logically equivalent sentences can refer to different states of affairs and therefore have different semantic properties. All tautologies are true in all possible worlds, yet they can refer to different states of affairs. A number of semantic theories assume that sentences refer to states of affairs. These theories provide principles that determine the reference of complex sentences on the basis of their parts (see Fine 2012: 234; see also Rumfitt 2015: 160–7).