Conditionals and conditional reasoning have been a long-standing focus of research across a number of disciplines, ranging from psychology through linguistics to philosophy. But almost no work has concerned itself with the question of how hearing or reading a conditional changes our beliefs. Given that we acquire much-perhaps most-of what we believe through the testimony of others, the simple matter of acquiring conditionals via others' assertion of a conditional seems integral to any full understanding of the conditional and conditional reasoning. In this paper we detail a number of basic intuitions about how beliefs might change in response to a conditional being uttered, and show how these are backed by behavioral data. In the remainder of the paper, we then show how these deceptively simple phenomena pose a fundamental challenge to present theoretical accounts of the conditional and conditional reasoning -a challenge which no account presently fully meets.
IntroductionEach day we encounter conditional sentences: sentences, most typically, of the form "If P, (then) Q". Conditionals can make assertions: "If people smoke, they have a higher risk of lung cancer." They can give advice or warnings: "If you stand here (there), you'll have a better (worse) view." They can express promises-"If I'm free, I promise I'll come to your party"-and bets-"If Liverpool and Tottenham make the final, I bet you £50 Liverpool will win." They can make claims about worlds that turned out differently from our own-"If Oswald hadn't killed Kennedy, someone else would have"-or that may yet come to pass-"If a sane candidate were to win, we might not be doomed." In short, conditionals are a flexible way to communicate information, or perform speech acts, under uncertainty.When we receive such conditionals, we must interpret them and we may change our beliefs. These sentences seem natural: we seem to understand them effortlessly and, where appropriate, assimilate their content seamlessly. And yet it remains somewhat mysterious how we change our beliefs when we encounter a conditional. This paper pursues the question of how we change our beliefs in response to conditional assertions. It will focus on indicative conditionals: conditional sentences in the indicative mood or, roughly, in which tense has the usual temporal meaning: present means present; past means past; and so on (Declerck & Reed, 2001). Another key type of conditional is the subjunctive or counterfactual conditional. To see the difference, compare these examples from