Includes bibliographical references and index.

"This chapter grapples with the challenges of defining depression, including challenges that arise from our imprecise use of language. Depression is at its core a kind of mood state. Mood states organize our minds and our bodies and motivate us to pursue goals. It is possible to understand depression by focusing on the scientific principles that explain why humans and other organisms have mood. A key goal is to become a more educated consumer of one's own mood and to understand the forces that operate on mood more generally. This framework can allow us to understand why people become depressed, why depression has occurred over human history, and why depression might be epidemic in some periods in human history, including the present day"--