Intentional, conscious speech is composed of both scientific and artistic components and thus requires instruction from both perspectives. Teachers often employ nonvalidated propositions--derived from personal and vicarious experience, results of scientific investigations, or folk wisdom--to advise students on speech performance. Care must be taken in translating scientific findings into a technology of speech improvement. But regardless of where teachers derive critical guidelines, the proper use of criticism requires a scientific approach characterized by the development and testing of hypotheses about the effect of particular changes and positively defined outcomes. The student speaker represents a population of behaviors, each of which is amenable to some kind of experimental analysis. The task of the teacher is to derive sound advice from scientists who study general behavior as a basis for a particular hypothesis about the behavior of a given person in a given case. Application of an understanding of scientific investigation to the derivation of critical principles acts as a method of criticizing generalizations, so that even if a false or useless generalization is selected by the teacher, individually testing it with particular speakers can provide an empirical base for decisions about its utility. (HOD)