As background to an argument for purposive reintegration of discourse study, this paper examines the concept of explanatory discourse as it developed within the Western rhetorical tradition. Following a discussion of the rise of a rhetoric of explanation, the first section cites the roots of the explanatory pedagogy developing during the first four decades of the nineteenth century: (1) the opening up of rhetorical theory implied by Hugh Blair and George Campbell, (2) the largely explanatory nature of pulpit oratory, (3) the growth of courses that treated writing as a discipline separate from speaking, and (4) the importation from secondary level textbooks of the concept of separate explanatory and argumentative themes. The second section of the paper begins with an examination of the rhetorical theorist, Henry Day, and then turns to the tradition that Day was up against--Alexander Bain's modes of discourse. The paper concludes with an examination of four elements that affected the development of twentieth century explanatory rhetoric: the decay of the modal classification of discourse and the failure of any other taxonomy to supplant it; the concurrent popularity of the Deweyite movement; the complete acceptance after 1910 of Fred Newton Scott and Joseph Denney's means of developing paragraphs; and the tendency for expository genres to split off from general composition once their pedagogy had achieved a sufficiently developed formal character, as has been the case for journalism and technical and business writing. (HOD)