Akademik

Sometimes known as subjunctive conditionals, although the terms do not exactly coincide. A counterfactual is a conditional of the form ‘if p were to happen q would’, or ‘if p were to have happened q would have happened’, where the supposition of p is contrary to the known fact that not-p . Such assertions are nevertheless useful: ‘if you had broken the bone, the X-ray would have looked different’, or ‘if the reactor were to fail, this mechanism would click in’ are important truths, even when we know that the bone is not broken or are certain that the reactor will not fail. It is arguably distinctive of laws of nature that they yield counterfactuals (‘if the metal were to be heated, it would expand’), whereas accidentally true generalizations may not. It is clear that counterfactuals cannot be represented by the material implication of the propositional calculus, since that conditional comes out true whenever p is false, so there would be no division between true and false counter-factuals.

Although the subjunctive form indicates a counterfactual, in many contexts it does not seem to matter whether we use a subjunctive form, or a simple conditional form: ‘if you run out of water, you will be in trouble’ seems equivalent to ‘if you were to run out of water, you would be in trouble’. In other contexts there is a big difference: ‘if Oswald did not kill Kennedy, someone else did’ is clearly true, whereas ‘if Oswald had not killed Kennedy, somebody else would have’ is most probably false.

The best-known modern treatment of counterfactuals is that of David Lewis, which evaluates them as true or false according to whether q is true in the ‘most similar’ possible worlds to ours in which p is true. The similarity-ranking this approach needs has proved controversial, particularly since it may need to presuppose some notion of the same laws of nature, whereas part of the interest in counterfactuals is that they promise to illuminate that notion. There is a growing awareness that the classification of conditionals is an extremely tricky business, and categorizing them as counterfactual or not may be of limited use.