According to william james, the purpose of psychology was to

"study the function of behavior".

Explanation:

The pioneer of Functional Psychology developed in the United States was the American William James. Its importance to psychology has been questioned because of its interest in mystical subjects such as telepathy, clairvoyance, spiritualism, and communication with the dead. He had a real distaste for the method of experimentation in psychology and did little work in this area. For him, the aim of psychology was to study the function of human behavior.

James founded no new thought stream or school of psychology, his attitude as a researcher was nothing experimental, as they wanted to make it appear by associating his form of psychology with the form of experimental psychology. He did not found functional psychology, but clearly and effectively presented his ideas within the functionalist atmosphere pervaded with American psychology, influencing the functionalist movement and inspiring later generations of psychologists.