Jean-Paul Sartre (1905-1980) is considered the most popular existentialist of the twentieth century and arose from the third force movement in psychology. The Third Force movement of the twentieth century consisted of a diverse set of psychologists and philosophers who did not all share universal principles, but all reacted to the new orientation of psychology, which differed from behaviorism in that it did not reduce psychological processes to reactions to the mechanical laws of physiological events but recognized the mediating and active role of the mind (Brennan, 2003). Despite the diversity of the Third Force movement, there were commonly held opinions among its pioneers. One of these views was the focus on personal freedom and responsibility in terms of decision making and realizing one's potential (Brennan, 2003). The mind was considered active and dynamic and a place where an individual could express their uniquely human capacities for cognition, will, and judgment (Brennan, 2003). There was an emphasis on the self and a recognition of the striving of human beings towards individually defined personality development. Existentialism holds that an individual is free to define his or her life course through his or her choices and decisions, but individuals are responsible for the consequences of their personal choices and decisions, and therefore freedom is a burdensome source of distress (Brennan, 2003). .At the heart of Jean-Paul Sartre's view was that existence precedes essence. This contrasted with Aristotelian and Scholastic views that individual existence is an expression of essence or being (Brennan, 2003). Sartre instead believed that existence defined the essence of an individual as he is... in the middle of paper... as he is in reality. Part of ACT's committed action involves clients setting goals in specific areas along their valuable life journey, and then acting on the goals while anticipating psychological barriers (Hayes, 2004). Clients are encouraged to take responsibility for the patterns of actions that result from their committed action goals. Sartre states that we have a total and profound responsibility in every decision we make, so much so that if we decide to commit ourselves to “truthfulness”, we will impose a standard of truth on ourselves and we will have to take responsibility for this standard. Finally, the idea that we can only rely on ourselves and must accept our own cognitions to increase our psychological flexibility in ACT, is similar to Sartre's idea that we have responsibilities only to ourselves and must guide ourselves through our ability to make decisions and choices..