The authors present their research into the problem of extending the classical logics to assign the value “unknown” to those formulas whose truth values are truly unknown. The book extends the usual first-order logic to include an epistemic operator, forming a language they call KL. Properties of knowledge are examined in detail to capture the semantics of this new logic. The operations TELL and ASK are defined for the interaction language KL, and these concepts are then related to autoepistemic logic. The first eight chapters of the book are basic, and the later chapters apply these concepts to different views of nonmonotonic logic.