Ruth M.J. Byrne
“How people reason with counterpossibles”
I discuss the results of three experiments designed to examine how people reason with counterpossibles (counterfactual conditionals with impossible antecedents). The experiments examine a range of counterpossibles, including (a) those that seem non-vacuously true such as “if lakes were made of bleach people would not swim in them” (which appears to be true whereas “if lakes were made of bleach people would swim in them” appears to be false), (b) counterpossibles that seem vacuously true, such as “If lobsters had been birds, they would have made nests” (which appears to be true but so does “If lobsters had been birds, they would not have made nests”), and (c) ones that seem false, such as “If houses had been made of spaghetti, their engines would have been noisy”. Participants made judgments about whether such counterpossibles were true or false (Experiments 1 and 2), and conditional inferences of modus ponens and tollens, denial of the antecedent and affirmation of the consequent (Experiment 3). The experiments show that participants distinguish between counterpossibles that are true and those that are false, and they show a tendency to make more inferences such as modus tollens and denial of the antecedent (as they do for counterfactuals) only from counterpossibles that are true, not from those that are false. The implications of the results are discussed for the debate about whether counterpossibles appear to be non-vacuously true merely as a result of cognitive heuristics.