US psychologist Abraham Maslow’s A Theory of Human Motivation is a classic of psychological research that helped change the field for good. Like many field-changing thinkers, Maslow was not just a talented researcher, he was also a creative thinker - able to see things from a new perspective and show them in a different light. He studied what he called exemplary people such as Albert Einstein, Jane Addams, Eleanor Roosevelt, and Frederick Douglass rather than mentally ill or neurotic people. 

Maslow generated new ideas, forging what he called "positive" or "humanistic psychology". His argument was that humans are psychologically motivated by a series of hierarchical needs, starting with the most essential first. His theories parallel many other theories of human developmental psychology, some of which focus on describing the stages of growth in humans.