Getting people to do their best work, even in trying circumstances, is one of managers’ most enduring and slippery challenges. Indeed, deciphering what motivates us as human beings is a centuries-old puzzle. Some of history’s most influential thinkers about human behavior—among them Aristotle, Adam Smith, Sigmund Freud, and Abraham Maslow—have struggled to understand its nuances and have taught us a tremendous amount about why people do the things they do.

A version of this article appeared in the July–August 2008 issue of Harvard Business Review.