The chapter to follow starts with a quote from Sir Francis Bacon. This preface can do no better than another quote from that famous gentleman. It was a singular occasion where he departed somewhat from his customary modesty, the place being the Proem to "The Advancement of Learning", around 1600-1605. "For myself, I found that I was fitted for nothing so well as for the study of Truth; as having a mind nimble and versatile enough to catch the resemblances of things (which is the chief point), and at the same time steady enough to fix and distinguish their subtler differences; as being gifted by nature with desire to seek, patience to doubt, fondness to meditate, slowness to assert, readiness to reconsider, carefulness to dispose and set in order; and as being a man that neither affects what is new nor admires what is old, and that hates every kind of imposture. So I thought my nature had a kind of familiarity and relationship with Truth." This statement clothes William K. Estes like the proverbial glove. That nature has arguably led to some of the most valuable contributions to psychology in the twentieth century. It has further guided several generations of experimental and theoretical psychologists through to their Ph.D. degrees and fruitful careers in research and teaching. There seems little doubt that Kay C. Estes has been an inextricable part of the 'Estes equation' over much of their professional lives (see, e.g., Estes in Lindzey, 1989). Together they form a dynamic duo in psychology if there ever was one. For these reasons, the subsequent chapter is dedicated to the both of them. May they continue their tradition of profound involvement and general invigoration of the science of psychology for many years to come.