Andrew Marr explores the limits of science and art in this week's Start the Week. The philosopher and neuroscientist Raymond Tallis mounts an all-out assault on those who see neuroscience and evolutionary theory as holding the key to understanding human consciousness and society. While fellow scientist Barbara Sahakian explores the ethical dilemmas which arise when new drugs developed to treat certain conditions are used to enhance performance in the general population. And the gerontologist Aubrey de Grey looks to the future when regenerative medicine prevents the process of aging.