It refers to the phenomenon that individuals can only respond to conditional stimuli after they have been matched with unconditional stimuli for many times. Its main principles include that conditioned stimulus must occur at the same time or near the same time, and conditioned stimulus must occur first.

If the stimulus is not strengthened, it will fade or even disappear. In the initial stage of the reaction, it may not be clear that the same response to similar conditioned stimuli is called generalization. Being able to clearly distinguish that you only respond to conditional stimuli and have no response to similar stimuli is called differentiation.

When he studied the phenomenon of digestion, he observed the dog's saliva secretion and reaction to food. The experimental method is as follows: show the dog food and measure its saliva secretion.

In this process, he found that if the food is repeatedly given a neutral stimulus, that is, a stimulus that does not automatically cause saliva secretion, such as ringing a bell, the dog will gradually learn to secrete saliva without food.

A stimulus that was originally neutral, plus a stimulus that could have caused a certain reaction, will make animals learn to respond to that neutral stimulus, which is the basic content of classical conditioned reflex.