Final answer:
The 'third force' in psychotherapy is the humanistic-existential therapy, which focuses on growth and self-actualization, diverging from psychoanalytic therapy and behaviorism. Pioneered by figures like Maslow and Rogers, it emphasizes free will and the positive potential of humans.
Step-by-step explanation:
The psychological perspective that introduced the concept of the third force in psychotherapy is the humanistic-existential therapy. This approach emerged as a reaction to the perceived limitations of psychoanalytic therapy and behaviorism, advocating for an individual's potential for growth and self-actualization. The philosophical underpinnings of this perspective include a focus on free will, self-determination, and the innate capacity for good within all humans. Key figures such as Abraham Maslow and Carl Rogers were instrumental in shaping this perspective, with concepts such as Maslow's hierarchy of needs highlighting the journey towards self-actualization. Humanistic therapists aim to provide a supportive environment where clients can explore their potential and strive towards becoming the best versions of themselves.