Reasoning with Exceptive Conditionals: The case of ‘Except If’

Résumé 0

"In this paper we outline a mental model theory (Johnson-Laird & Byrne, 1991, 2002) of reasoning about the conditional except if. We report two experiments showing that the exceptive conditional except if exerted certain forms of semantic modulation and determined the inferences that individuals draw in an inference task (Experiment 1) and in a truth table task (Experiment 2). In Experiment 1 we found that there were no reliable differences between the percentage of modus ponens and modus tollens with the conditional except if but that the differences were reliable with the conditional if not, then. In Experiment 2, participants selected the possibilities ‘A & not-B’ and ‘not-A & B’ more frequently than the possibilities ‘A & B’ and ‘not-A & not-B’ with the conditional ‘B except if A’, but they selected the possibility ‘not-A & B’ more frequently than other possibilities (‘A & not-B’, ‘A & B’, ‘not-A & not-B’) with the conditionals ‘B if not-A’ and ‘if not-A, B’. The implications of these results are discussed in the context of recent psychological and linguistic theories in respect of the meaning of except if."