Martin Seligman introduced positive psychology, which focuses on human strengths and well-being rather than just weakness and illness. He developed the Authentic Happiness Theory, which defined three types of happy lives: pleasant, engaged, and meaningful. This led to his Well-Being Theory, which defines five elements of well-being: positive emotion, engagement, relationships, meaning, and accomplishment. The goal is to increase these elements and achieve a state of flourishing. Seligman's research uses surveys to measure strengths and well-being in order to study positive psychology interventions.