Jean-Paul Sartre (1905-1980) is considered the most popular existentialist of the 20th century and emerged from the Third Force movement in psychology . The third force movement of the 20th century consisted of a diverse set of psychologists and philosophers who did not all share universal principles, but who all responded to the new direction in psychology, which was distinguished from behaviorism in that it did not reduce psychological processes to reactions. to the mechanical laws of physiological events but recognizes the mediating and active role of the mind (Brennan, 2003). Despite the diversity of the Third Force movement, its pioneers shared common views. One of these views was the emphasis on personal freedom and responsibility in terms of decision-making and realizing one's potential (Brennan, 2003). The mind was seen as an active and dynamic place and a place where an individual could express their unique human abilities of cognition, will, and judgment (Brennan, 2003). The emphasis was on the self and recognition of humans' efforts toward individually defined personality development. Existentialism holds that an individual is free to define their life course through their choices and decisions, but that individuals are responsible for the consequences of their personal choices and decisions, and that freedom is therefore a heavy source of anxiety (Brennan, 2003). At the heart of Jean-Paul Sartre's ideas was that existence precedes essence. This contrasts with Aristotelian and scholastic views that individual existence is an expression of essence or being (Brennan, 2003). Instead, Sartre believed that existence defines the essence of an individual as he or she actually is. Part of ACT's committed action involves clients setting goals in specific areas along their valued life path and then taking action on those goals while anticipating psychological barriers (Hayes, 2004 ). Clients are encouraged to take responsibility for the action patterns that result from their committed action goals. Sartre states that we have total and profound responsibility for every decision we make, such that if we decide to commit to "truthfulness", we will impose a standard of truth on ourselves and must take responsibility for that standard. Finally, the idea that we can only rely on ourselves and must accept our cognitions to increase our psychological flexibility in ACT is similar to Sartre's idea that we only have responsibility to ourselves ourselves and must guide ourselves through our ability to make decisions and choices..