This study describes the philosophical and psychological assumptions on which radical changes in the teaching of composition might be based; presents a program for making those changes; and tests the effectiveness of that program. It is predicted that if composition students learn to use the tagmemic heuristic model as a plan for guiding inquiry, their writing will reflect three basic changes: (1) Students will examine data more thoroughly; (2) Students' essays will contain fewer conceptual gaps; and (3) Students will solve problems more adequately. Essays were scored and the predictions were tested by the following questions: Did the post-test essays show an increase in the kinds of intellectual operation performed?; Was there a decrease in the number of conceptual gaps?; and Was there an increase in the essayist's use of evidence? All questions were answered affirmatively. The results of this study have significant implications for future testing of complex linguistic behavior, for rhetorical theory and the teaching of composition, and for educational theory and practice. (Author/LS)