The attempts to clarify (purify) the conceptual foundations of family therapy by means of “epistemology” have bred excitement, boredom, irritation and confu¬sion. In the belief that at least the confusion can be alleviated, the present paper is offered as a study guide and something of a Rosetta Stone for translating the work of Gregory Bateson and Humberto R. Maturana. The paper demonstrates that Maturana’s work is highly compatible with that of Bateson. In addition, several major points of contrast are argued: (1) Maturana’s concept of structure determinism is an explicit ontological claim which directly implies an episte¬mology, whereas Bateson delineated an epistemology, but never clearly developed a corresponding ontology; (2) structure determinism is a more general concept than Bateson’s concept of “mind” (i.e., cybernetic epistemology); (3) structure determinism deletes the remnants of objectivity from Bateson’s theory (i.e., “the difference that makes a difference”); and (4) Maturana’s concept of instructive interaction is a more general, nonsystemic version of what Bateson meant when he used the term “epistemological error.” Finally, it is claimed that the emphasis on epistemology has distracted proponents and detractors alike from the essential message of Bateson and Maturana: social systems and all human endeavor must be understood in light of our existence as biological entities that are coupled to a medium. The biological ontology implicit in Bateson’s writings and explicitly delineated in Maturana’s may (at long last) provide a sound foundation for the social and behavioral sciences.