One of the fundamental theories in the history of psychology was made by a physiologist, not a psychological experimenter. In the early 1890s, the Russian physiologist Ivan Pavlov, studying the physical workings of the stomachs of dogs, devised a way of collecting and measuring their saliva to discover how it related to digestion. Instead, he noticed that saliva wasn’t just a reaction to the presence of food. Dogs also swallowed saliva in anticipation of food, as well as thinking about food as a psychological stimulus.
Changing the track of these experiments to explore this phenomenon he discovered the principle of conditioning, a cornerstone of the theory of behaviorism – the approach that was to rule psychological science for the next fifty years. Pavlov’s experiments also showed that something as seemingly complex as animal behavior can be studied by experiment in the laboratory under controlled conditions, not simply observed in the natural world.