An approach to the assessment of classroom thoughtfulness is presented. The approach recognizes the importance of in-depth knowledge, intellectual skills, and dispositions; it also emphasizes general qualities of discourse such as teachers posing higher order challenges and students giving reasons. The approach is contrasted with those that attempt to prescribe highly specific instructional procedures for teaching discrete thinking skills or specific bodies of content. The classroom observation scheme was used to assess levels of thoughtfulness in 39 diverse 9th grade social studies classes in 7 midwest high schools during the academic year 1988-1989. At the end of the year, students read two pages of background information on a constitutional issue and completed a written exercise asking them to state and to defend their position. Although teachers had not prepared students for such an exercise, the persuasiveness of student reasoning on the constitutional issues was strongly associated with the level of classroom thoughtfulness to which students were exposed, even after controlling for student scores on a pre-test of social studies knowledge, a pre-test of writing, student grade point average, race, sex, parents' education, and the racial and ability composition of the class. The design did not allow demonstration of a clear causal effect, but the evidence is consistent with the conclusion that general qualities of classroom discourse over a wide range of subjects affect student performance in higher order thinking. A 31-item bibliography is included. (Author/DB)