  Another theory of possible worlds has been developed by David Armstrong. Armstrong has an actualist combinatorialist theory of possibility, according to which what is possible is determined by appropriate combinations of actual elements (particulars and universals). The basic notion in Armstrong's theory of possibility and possible worlds is that of an atomic state of affairs. A state of affairs brings together a particular and a universal (if the universal is a property), or some particulars and a universal (if the universal is a   relation).[27] 