PERCEIVED NECESSITY EXPLAINS THE DISSOCIATION BETWEEN LOGIC AND MEANING - THE CASE OF ONLY IF

The purpose of this article was to investigate why sentences of the fo rm ''p only if q'' often appear to have a different meaning than sente nces of the form ''if p, then q,'' despite their logical equivalence. The results of 3 experiments indicate that when ''if...then'' statemen ts were equated with respect to necessity and temporal relations, diff erent pragmatic relations (such as permission, causation, etc.) elicit ed similar ''only if'' judgments. However, different necessity relatio ns elicited different ''only if'' judgments, regardless of the type of pragmatic relation expressed in the ''if then'' statement. These data suggest that ''only if'' judgements are primarily mediated by necessi ty and temporal relations and that pragmatic contexts may play a more indirect role, such as in the interpretation of necessity and temporal relations. Suggestions for how these findings might be incorporated i nto pragmatic schema theory (P. W. Cheng & K. J. Holyoak, 1985; P. W. Cheng, K. J. Holyoak, R. E. Nisbett, & L. M. Oliver, 1986) and mental models theory (P. N. Johnson-Laird & R. M. J. Byrne, 1991) are discuss ed.