Irreducibly first-person thought, often called essentially indexical thought or de se thought, plays an essential role in explaining action and yields a distinctive form of self-knowledge.. While prominent defenses of the irreducibility of first-person thought (e.g., those of John Perry or David Lewis) extend only to self-directed thought, I develop analogous arguments for thought about other agents, showing that irreducibly second-person thought plays an analogous role in explaining interaction and yields a distinctive form of knowledge about other agents. On my account, irreducibly second-person thought represents its object as an agent with whom I am engaged in interaction, and second-person thoughts are best understood--extending the account of Francois Recanati--as "mental files" containing distinctively second-person information. Recognizing an agent as a partner in interaction is characteristic of a distinctively social form of thought, the capacity for which lies at the root of interpersonal morality.