Joseph Priestley, in his "A Course of Lectures on Oratory and Criticism," developed a psychological theory of style. The "Course" covers three main topics: traditional rhetorical arts of invention, arrangement, and style. Borrowing from the ideas of David Hartley, the association psychologist; Joseph Addison, the aesthetician; and Adam Smith, the moralist--all of whom offer Priestley a psychology of the human mind, Priestley contributed to discussions of style in 18th century rhetoric by synthesizing and expanding upon the thought of the best thinkers of his day and by using Lockean associationism to explain stylistic effects. Priestley's rhetoric is one of the most successful attempts to base a theory of style on association psychology. By doing so, Priestley rejected the classical concepts of high, middle, and low style and replaced them with a theory that emphasized the psychological impact language can have on readers and listeners. Language becomes the medium through which an individual mind can communicate with another, and style becomes the group of techniques writers and speakers use to gain assent, which Priestley conceived of as a psychological process. (RAE)