 Modern treatments of the logic of knowledge and belief grew out of the work of philosophers and logicians writing from 1948 through the 1950s. Rudolf Carnap, Jerzy Łoś, Arthur Prior, Nicholas Rescher, G.H. von Wright and others recognized that our discourse concerning knowledge and belief admits of an axiomatic-deductive treatment. Among the many important papers that appeared in the 1950s, von Wright’s seminal work (1951) is widely acknowledged as having initiated the formal study of epistemic logic as we know it today. Von Wright’s insights were extended by Jaakko Hintikka in his book Knowledge and Belief: An Introduction to the Logic of the Two Notions (1962). Hintikka provided a way of interpreting epistemic concepts in terms of possible world semantics and as such it has served as the foundational text for the study of epistemic logic ever since.