This study investigated the untested assumption that people's everyday reasoning reflects different understandings of the nature of knowledge and knowing. The epistemological stances of 180 prospective jurors were assessed with an interview that probed the nature and source of the discrepant knowledge claims of two historical accounts. The researchers derived global epistemological levels from these interviews. The jurors also heard trials and offered justifications for their verdict choices. The researchers assessed these justifications for whether subjects could use various reasoning skills successfully. Epistemological level, but not educational level, age, or gender, predicted juror-reasoning skills and degree of certainty about verdict choice. Epistemological level, mediated by the juror-reasoning skills, was a better predictor of general argument skill than certainty about verdict choice and the amount of evidence used in arguing for a verdict. The results indicate that epistemological understandings underlie specific juror-reasoning skills and overall argument ability.