 Like most contemporary counterfactual theories, Lewis’s theory employs a possible world semantics for counterfactuals. Such a semantics states truth conditions for counterfactuals in terms of similarity relations between possible worlds. Lewis famously espouses realism about possible worlds, according to which non-actual possible worlds are real concrete entities on a par with the actual world (Lewis 1986e). However, most contemporary philosophers would seek to deploy the explanatorily fruitful possible worlds framework while distancing themselves from full-blown realism about possible worlds themselves (see the entry on  possible worlds).