A major problem in ethnographic description may be summed up as the search for ways to disentangle folk from analytical models. Knowledge-based systems have contributed to the development of formal structures for the manipulation of symbols associated with particular physical and conceptual phenomena. Importantly, their output can be interpreted by experts in the domain. This provides evaluation procedures for models elaborated jointly by analysts and informants. This paper describes an interactive system in which knowledge is represented as production rules in a format derived from the theory of formal languages. Modus ponens and modus tollens are explained and compared to derivation schemata in first-order predicate logic. The results of an application to the study of North Indian tabla drumming are assessed. We conclude that (1) knowledge represented at a low theoretical level fails to descriminate between the input from informants and the intuitive assumptions of analysts, (2) experimental procedures can be improved considerably if the system is designed to perform automated knowledge acquisition (using probabilistic grammars and inductive learning).