Cetana and the Dynamics of Volition in Theravada Buddhism

What do the scriptures of Thravada Buddhism have to say about the most basic psychological processes through which alternatives are assessed, purposes are developed, and goal-oriented acts are initiated? How can Theravada make volitional endeavour central to Buddhist practice, while denying the existence of a self who wills? How can the text emphasize ethical striving, and yet uphold the principle that all physical and mental acts arise through causes and conditions.