Stimulus Response Theory
Stimulus Response Theory is a concept in psychology that refers to the belief that behavior manifests as a result of the interplay between stimulus and response. In particular, the belief is that a subject is presented with a stimulus, and then responds to that stimulus, producing "behavior" (the object of psychology's study, as a field). In other words, behavior cannot exist without a stimulus of some sort, at least from this perspective.
When one thinks of Stimulus Response Ttheory, one can't help but think of classical conditioning. Of course, classical conditioning presents the concept of stimulus and response very succinctly, as it demonstrates the way that a stimulus can evoke a predictable and consistent response in a subject with very little effort. And when one thinks of classical conditioning, one can't help but think of Ivan Pavlov and his dogs.
Pavlov was a Russian researcher working near the turn of the century. In his famous experiments, he conditioned a group of dogs to salivate when they heard a dinner bell ring. This was achieved as follows. To begin with, Pavlov had an unconditioned stimulus, the dog's food. When presented with this unconditioned stimulus, the dogs would salivate, naturally, as an unconditioned response. Pavlov began to ring a bell whenever his dogs were fed, and over time found that the bell alone, without the presence of food, could reduce the expected response of salivation. Overtime, the salivation had become a conditioned response to the conditioned stimuli of the ringing bell.