How do we determine right from wrong? Conscience illuminates the answer through science and philosophy. In her brilliant work Touching a Nerve, Patricia S. Churchland, the distinguished founder of neurophilosophy, drew from scientific research on the brain to understand its philosophical and ethical implications for identity, consciousness, free will, and memory. In Conscience, she explores how moral systems arise from our physical selves in combination with environmental demands. All social groups have ideals for behavior, even though ethics vary among different cultures and among individuals within each culture. In trying to understand why, Churchland brings together an understanding of the influences of nature and nurture. She looks to evolution to elucidate how, from birth, our brains are configured to form bonds, to cooperate, and to care. She shows how children grow up in society to learn, through repetition and rewards, the norms, values, and behavior that their parents embrace. Conscience delves into scientific studies, particularly the fascinating work on twins, to deepen our understanding of whether people have a predisposition to embrace specific ethical stands. Research on psychopaths illuminates the knowledge about those who abide by no moral system and the explanations science gives for these disturbing individuals. Churchland then turns to philosophy—that of Socrates, Aquinas, and contemporary thinkers like Owen Flanagan—to explore why morality is central to all societies, how it is transmitted through the generations, and why different cultures live by different morals. Her unparalleled ability to join ideas rarely put into dialogue brings light to a subject that speaks to the meaning of being human.