In the early part of the twentieth century conditioning had been considered to consist solely of the kind discovered and espoused by Pavlov, and it was not until the 1920s and 1930s that theorists began to consider that there might be two fundamentally different types of basic associative learning. The dichotomy between Pavlovian and instrumental responding was arguably first discussed by Konorski and Miller (1937) and then later popularized by Skinner (1938) in his classic book Behavior of Organisms. Skinner’s position on the differences between Pavlovian and instrumental conditioning was particularly radical: not only were they different learning procedures, but they also possessed different biological and adaptive functions and thus, by implication, were manifestations of different underlying learning mechanisms. Skinner’s view was that Pavlovian conditioning represented the adaptive mechanism by which the more reflexive, autonomic components of behavior were modified, whereas instrumental conditioning was the process by which the more integrated, skeletally-mediated and “voluntary” behaviors of an organism were learned. This dichotomy persisted until well into the 1960s, when it began to become clear that integrated skeletally-based responses could be acquired and modified by Pavlovian contingencies (for example, autoshaping, see p. 27), and that certain autonomic and visceral reponses were directly susceptible to instrumental contingencies (cf. Miller, 1969).