ABSTRACT
The topic of equivalence relations, sometimes referred to as stimulus equivalence or equivalence classes, is relatively new to behaviour analysis. There is reason to believe that the concept is important for our understanding of behaviour, but the term equivalence is occasionally beginning to be used somewhat more broadly than is perhaps useful. So I have chosen this occasion to go back over some of the basics and, at the same time, to discuss a fundamental and still unsolved problem: Where do equivalence relations come from?
