Epistemologists have argued that there are three basic sources of belief: perception, testimony and inference. These three belief sources correspond directly to the way in which many languages mark statements morphologically for sources of evidence for the statements (evidentiality). In this paper, we connect generalizations from the fields of epistemology and evidentiality. We also introduce a new method for investigating how reliable people find different types of evidence to be. A study based on this method indicates that speakers of English rank different sources of evidence according to the same criteria that govern the use of grammaticalized evidential marking.