Possibilities and human reasoning

Abstract

This article describes how human reasoning differs from standard logics. It tells the story of three sorts of inference, for example: the possibility of rain implies the possibility of no rain; a forecast of rain or snow implies the possibility of rain and the possibility of snow; and a forecast of frost does not imply a forecast of frost or snow or both. People accept these inferences, which each violate the semantics of standard logics. But, they are predictions from the theory that assertions refer to mental models of possibilities that each hold in default of knowledge to the contrary. The moral of the story is that inconsistencies in human reasoning, unlike those in standard logics, have only local consequences.