Ivan Pavlov

Commuting stinks, and it is made more tolerable by books-on-CD, such as Psychology of Human Behavior.

Most people are familiar with the name Pavlov and the concept of "Pavlovian response" or "Pavlovian conditioning". According to the book-on-CD Pavlov, who won his Nobel prize for physiology before he started his experiments on dogs, stumbled on to his subject. He used machines to feed dogs so as to study how the digestive system performs its work. Salivation is part of the digestive system, and he and his assistants would collect the saliva as they would collect bile and acid. One day the machine malfunctioned, but still produced the standard click-and-whirr noises, and the dogs still salivated.

Some definitions are in order:
Unconditioned stimulus: a naturally occurring stimulus.
Unconditioned response: a naturally occurring response. When food enters an animal's mouth (the unconditioned stimulus) salivation occurs (the unconditioned response). No learning is necessary for this response, it is "built-in".
Conditioned stimulus: an artificially occurring stimulus, such as a bell being sounded at the same time that food is presented.
Conditioned response: a response generated from the conditioned stimulus (often the same as the unconditioned stimulus).

There are two types of conditioning: classical and operant. Classical conditioning may be approximated as involuntary actions. For example, a sudden noise or flash of light may generate faster heartbeats, and producing such noises or light for the purpose of generating heartbeats would be called classical conditioning. Operant conditioning can be approximated as the study of voluntary actions: does increasing pay generate more or higher-quality work from employees?