Cumulative Record. Chapter 28: The Operational Analysis of Psychological Terms. Quote 17

The question “What is length?” would appear to be satisfactorily answered by listing the circumstances under which the response “length” is emitted (or, better, by giving some general description of such circumstances). If two quite separate sets of circumstances are revealed, then there are two responses having the form “length,” since a verbal response-class is not defined by phonetic form alone but by its functional relations. (pp. 418-419)