Skinner traces the historical origins of the reflex concept, noting that early mechanistic analogies—such as Rene Descartes’s comparison between hydraulics and muscle movement—challenged the idea that behavior was purely spontaneous. Over time, experimental findings, such as a salamander’s tail moving when stimulated, demonstrated that some behavior could be explained as a response to external events rather than inner will. This led to the formalization of the stimulus-response relationship as a “reflex,” with measurable properties like latency and magnitude. As scientific understanding expanded, the scope of reflex action increased, gradually displacing inner-cause explanations.

While reflexes allow for precise prediction in certain cases—like the pupil contracting in response to light—they account for only a small fraction of total behavior. The principle gained broader significance with Ivan Pavlov’s discovery of the conditioned reflex, in which a neutral stimulus elicits a response through pairing with a stimulus. Pavlov replaced cage explanations like “psychic secretion” with precise, controllable conditions. He also identified the process of extinction, in which a conditioned stimulus loses its effect when reinforcement stops.

Skinner discusses the evolutionary “survival value” of both unconditioned and conditioned reflexes, while noting that not all conditioned responses are adaptive—some, like phobias or superstitions, result from accidental pairings of stimuli.