Counterfactual conditional

Counterfactual conditionals (also contrafactual, subjunctive or X-marked conditionals) are conditional sentences that describe what would have been true if circumstances had been different, typically when the antecedent is taken to be false or incompatible with what actually happened. In English they are often formed with a past or irrealis morphology, as in "If Peter believed in ghosts, he would be afraid to be here", and are standardly contrasted with indicative conditionals, which are generally used to discuss live or open possibilities.

The name subjunctive conditionals is sometimes preferred because counterfactuals are not always "contrary to fact" as the word "counterfactual" implies, but the name counterfactual conditionals is sometimes preferred because counterfactuals are not always grammatically subjunctive. Hence the name X-marking has been developed as a compromise, although it is newer in the literature; see § Terminology.

Counterfactuals are central topics in philosophical logic, formal semantics, and philosophy of language. In particular, several conditional logics have been developed specifically to study counterfactuals. Early work treated them as a challenge for the analysis of conditionals in terms of the material conditional, which would make all counterfactuals with false antecedents trivially true. Subsequent research developed a range of non-truth-functional accounts, especially possible world approaches such as Lewis's variably strict analysis and Stalnaker's closest-world semantics, as well as alternatives based on strict conditionals, causal models, and belief revision and the Ramsey test.

From a linguistic perspective, counterfactuality is closely tied to the grammatical marking of tense, aspect, and mood. Many languages use so-called fake tense and fake aspect to signal a counterfactual interpretation, sometimes analyzed as instances of more general X-marking that distinguishes these conditionals from their indicative or O-marked counterparts. Research in psychology and cognitive science studies how people understand and reason with counterfactual conditionals, how they differ from indicatives in comprehension and inference, and how they relate to more general patterns of counterfactual thinking and mental simulation.