Abraham Maslow, an influential psychologist, is best known for developing the hierarchy of needs, a theory which posits that human beings have specific needs that must be met in order to pursue higher levels of psychological fulfillment and self-actualization. His contributions to humanistic psychology emphasize the importance of positivity, creativity, and the potential within individuals, positioning humanism as a 'third force' in psychology alongside psychoanalysis and behaviorism. Maslow's hierarchy outlines five levels of needs, from basic physiological needs to self-actualization, highlighting that unmet lower-level needs can hinder personal growth and well-being.