Purpose-a cognitive process that defines life goals and provides personal meaning-may help explain disparate empirical social science findings. Devoting effort and making progress toward life goals provides a significant, renewable source of engagement and meaning. Purpose offers a testable, causal system that synthesizes outcomes including life expectancy, satisfaction, and mental and physical health. These outcomes may be explained best by considering the motivation of the individual-a motivation that comes from having a purpose. We provide a detailed definition with specific hypotheses derived from a synthesis of relevant findings from social, behavioral, biological, and cognitive literatures. To illustrate the uniqueness of the purpose model, we compared purpose with competing contemporary models that offer similar predictions. Addressing the structural features unique to purpose opens opportunities to build upon existing causal models of "how and why" health and well-being develop and change over time.