Proceedings of the Annual Meeting of the Cognitive Science Society

The inference of or-introduction, p, therefore p or q, is fundamental in classical logic and probability theory. Yet traditional research in the psychology of reasoning found that people did not endorse this inference as highly as other one- premise valid inferences. A radical response to this finding is to claim that or-introduction is in fact invalid. This response is found in the recent revision of mental model theory (MMT). We argue that this revision of the theory leads to a number of logical problems and counterintuitive consequences for valid inferences, and present an experiment extending recent studies showing that people readily accept or-introduction under probabilistic instructions. We argue for a pragmatic explanation of why the inference is sometimes considered odd. The inference is not odd when people reason from their degrees of belief.