 The main aim of belief revision theory is to say how you should revise your beliefs when you learn new information. Suppose you learn about the existence of a new planet, Algernon. How should \(K\) change when you learn this new fact, \(A\)? As long as \(A\) doesn’t contradict your existing beliefs, the standard view is that you should just add \(A\) to \(K\), along with everything that follows logically from the members of \(K\) and \(A\) together. We call the new set of beliefs \(K + A\): add \(A\) to \(K\) along with all that follows logically (Alchourrón, Gärdenfors, and Makinson 1985).