Final answer:
The 'third force' in psychotherapy is known as humanistic psychology, characterized by its positive view of human potential and emphasis on individual growth.
Step-by-step explanation:
The 'third force' in psychotherapy is humanistic psychology. Humanistic psychology emerged as a reaction to the perceived limitations of psychoanalysis and behaviorism. It emphasizes the potential for good innate in all humans, highlighting personal control, intentionality, and the innate capacity for self-directed change and growth. Pioneers of humanistic psychology like Abraham Maslow and Carl Rogers have been instrumental in advancing this perspective. Maslow's hierarchy of needs and Roger's client-centered therapy are key contributions that emphasize the importance of individual potential and health rather than just psychological disturbances.