The study of classical/Pavlovian conditioning began in the reflex tradition. Physiologist Pavlov was interested in the brain, and the conditioned salivation of dogs was his way of studying it. But, when released from restraint, his dogs showed much other activity in anticipation of the food powder about to be dropped into their mouths. A visitor to Pavlov's laboratory reports the reaction when a dog, trained to salivate at the sound of a metronome, was released: The animal at once approached the metronome, wagged its tail, barked, and behaved in every way as it might toward an individual who could feed it. But gastric expert Pavlov regarded this behavior as a distraction. His dogs were therefore restrained in a special harness so that their saliva could be collected more easily. Yet salivation is the least important thing they learned. But it is, of course, the thing most easily observed, measured, and understood in reflex terms. And salivation is directed by the autonomic, involuntary nervous system, not the voluntary, somatic one.

B. F. Skinner, father of operant conditioning, was a great admirer of Pavlov and emulated him in many respects. He followed Pavlov's approach to conditioning in one way but not another. First, unlike both Pavlov and independent discover of the operant method D. C. Grindley, he did not restrain his animals. This freedom allowed Skinner to discover schedules of reinforcement and led to a rich evolution of the operant method. But Skinner accepted Pavlov's belief that classical conditioning is a separate system, dealing not with operants (voluntary responses – although Skinner avoided that word) but respondents, involuntary responses like salivation, the flick of the nictitating membrane, and the conditioned emotional response (CER).

The behavior of the dog released from Pavlov's harness shows that it had learned a great deal more than salivation. It is this learning, which is to do with behavioral variation – the origins of operant behavior rather than its selection by reinforcement – that is the hugely important effect of Pavlovian conditioning.

Did the dog released from his harness also salivate to the sound of the metronome? Probably not; probably the dog did not salivate until it actually reached the instrument (or the food bowl). Several elegant experiments have established that autonomic responses such as salivation tend to occur at the last predictable moment before food delivery.