This chapter traces the distinction between knowledge and opinion from Plato to contemporary social science and shows how ancient thinking is linked to modern conceptualizations of health-related knowledge and its consequences for health behaviors. While Plato was concerned with how a human can distinguish his own knowledge from his opinions, and with the role that certainty plays therein, contemporary social science is concerned with differentiating humans' subjective and objective knowledge from an observer position. Elements of these distinctions find their way into a model of the complex relationships between health information seeking, subjective health knowledge, health literacy, and empowerment to explain health behavior. The sketch shows that ancient philosophy can help understand and conceptualize contemporary variable-oriented modeling.