REASONING FROM SUPPOSITIONS

Two experiments investigated inferences based on suppositions. In Expe riment 1, the subjects decided whether suppositions about individuals' veracity were consistent with their assertions-for example, whether t he supposition ''Ann is telling the truth and Beth is telling a lie'', is consistent with the premises: ''Ann asserts: I am telling the trut h and Beth is telling the truth. Beth asserts: Ann is telling the trut h''. It showed that these inferences are more difficult than ones base d on factual premises: ''Ann asserts: I live in Dublin and Beth lives in Dublin''. There was no difference between problems about truthtelle rs and liars, who always told the truth or always lied, and normals, w ho sometimes told the truth and sometimes lied. In Experiment 2, the s ubjects made inferences about factual matters set in three contexts: a truth-inducing context in which friends confided their personality ch aracteristics, a lie-inducing context in which business rivals adverti sed their products, and a neutral context in which computers printed t heir program characteristics. Given the supposition that the individua ls were lying, it was more difficult to make inferences in a truth-ind ucing context than in the other two contexts. We discuss the implicati ons of our results for everyday reasoning from suppositions, and for t heories of reasoning based on models or inference rules.