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The emergence of structure from undifferentiated beginnings has long been a fundamental problem in science. In biology, the issue was one of form versus function, and in psychology psychologists struggled with how infants make sense of, and consolidate, the flood of sensory input they are faced with. Although the concept of discriminative responding has proven useful in this regard, describing the emergence of structure which sometimes follows conditional discrimination procedures as stimulus equivalence has had important implications for subsequent research in the field.

Arising from the plethora of research on stimulus equivalence, the theoretical treatises of Sidman (1994), S. C. Hayes (1994), and Horne and Lowe (1996) have distinguished themselves quickly in a crowded field. As all three of the substantive positions appear to be developing parallel to each other, some history of the field as well as inherent shortcomings of each of the theoretical positions are discussed. Secondary theories and important new methodologies suggest where the field is or should be heading if we are to keep sight of our original goals.