This study of Campbell's ""Philosophy of Rhetoric"" (published 1776) is a historical and text-immanent interpretation setting out to substantiate the hypothesis that Campbell's rationale for explaining rhetorical impact is determined by the spirit of English empiricism. Campbell interprets David Hume's concept of 'vivacity' in terms of the idea of evidentia taken from Classical rhetoric theory. The study shows that evidentia is the category underlying not only ethos, logos and pathos but also embellishment and actio.