This book investigates the core question of whether personal identity over time is fundamentally a psychological or a biological phenomenon. Eric T. Olson, a philosopher specializing in metaphysics and the philosophy of mind, challenges the prevailing psychological continuity theories that dominate contemporary discourse. By utilizing a biological approach, he argues that human beings are essentially organisms, meaning that psychological states are neither necessary nor sufficient for persistence through time.

Experts in the field of metaphysics frequently cite this work as a primary text for the animalist position in the debate over personal identity. Readers often note the rigorous logical structure of the arguments, which serves as a standard counterpoint to Lockean psychological accounts.