Ivan Pavlov in the early 1900s demonstrated the ease with which a response could be classically conditioned, a phenomenon that would ultimately form the basis of behavior therapy. Pavlov repeatedly presented a bell to dogs just prior to the appearance of meat powder. The meat powder was an unconditioned stimulus because it naturally made the dogs salivate without any earlier learning. After repeatedly hearing the bell before receiving the meat powder, the dogs learned to salivate in response to the bell, prior to getting the meat. This indicated that the bell had become a conditioned stimulus to elicit salivation, and mouth watering was now a conditioned response to the bell.

Based on this discovery of the ...