I was privileged to have been associated in one form or another with the Harvard Pigeon Lab from September 1964, when I arrived as a graduate student, until my friend and colleague Gene Heyman moved out of William James Hall on June 30, 1998. After completing my graduate degree in 1968, I continued to live in Cambridge and was therefore able to continue my affiliation with the Pigeon Lab, frequenting the lab meetings to learn about the latest research and occasionally to present some ideas of my own. During the academic year 1981–1982, I was a full-time visiting scholar at the lab, regularly attending lab meetings as well as weekly conversations with Fred Skinner. These latter meetings, or ‘‘pow-wows’’ as Fred called them, were attended by a small group consisting of Will and Maggie Vaughan, Pere Julia, another visiting scholar at the time, and me, as well as occasional invited guest speakers (Skinner, 1983, p. 394). When I arrived at Harvard in 1964, the Pigeon Lab was about to move from the basement of Memorial Hall into William James Hall, and it had completed its transition from Skinner’s leadership to Herrnstein’s. Skinner had accepted a government Career Award and was retired from teaching and departmental responsibilities, although he remained Professor of Psychology. Not only was this a change in style, but it also meant a basic shift in research direction. For Skinner, the fundamental dependent variable in operant research was the absolute response rate of the free-operant response. He conceptualized response rate as closely associated with response probability, or response strength, the successor to the earlier reflex reserve.