In the same way that a child must be able to move his arms and legs before he can learn to walk, the child must physiologically be capable of producing and experiencing particular emotions before these emotions can be modified through learning. Psychologists have found that there arc two basic processes by which learning takes place. One kind of learning is called'' classical conditioning.' This occurs when one event or stimulus is continuously followed by a reward or punishment. It is through classical conditioning that a child learns to associate his mother's face and voice with happiness and love, for he learns that this person provides food and comfort. Negative emotions are learned in a similar fashion. The second kind of learning is called 'operant (动作的) conditioning. 'This occurs when an individual learns to do things that produce rewards in his environment and learns not to do things that produce punishments. For example, if a mother always attends to her baby when he cries and cuddles him until he is quiet, she may teach him that if he cries he will get attention from mother. Thus, the baby will learn to increase his crying in order to have his mother more. The main purpose of this passage is to ______.