Theories of learning have long emphasized the essential role of social factors in the development of early reasoning abilities. More recently, it has been proposed that the presentation of conflicting perspectives may facilitate young children's understanding of knowledge claims as potentially subjective--one of many possible representations of the world. This development in epistemological understanding has been proposed to be an important determinant of academic performance and is highly correlated with the ability to understand and produce sound argumentation in adolescents and adults. In a longitudinal study of children 7-8 years old, we assessed the effects of a 3-month philosophy class designed to engage children in dialogic interaction with peers. We examined the influence of this intervention on children's epistemological understanding and argumentation skills in 4 domains of knowledge: aesthetic, value, social, and physical. Participation in dialogic interaction in an elementary school classroom improved children's ability to construct their own and opposing arguments across domains and facilitated reasoning about the subjectivity of knowledge in the value domain.