“A person cannot direct his emotional life in the way he bids his motor system to reach for a cup. He cannot will himself to want the right thing or to love the right person or to be happy after a disappointment, or even to be happy in happy times. People lack this capacity not through a deficiency of discipline but because the jurisdiction of will is limited to the latest brain and to those functions within its purview. Emotional life can be influenced, but it cannot be commanded.”