 Until relatively recently, epistemic logic focused almost exclusively on propositional knowledge. In cases of propositional knowledge, an agent or a group of agents bears the propositional attitude of knowing towards some proposition. For example, when one says: “Zoe knows that there is a hen in the yard”, one asserts that Zoe is the agent who bears the propositional attitude knowing towards the proposition expressed by the English sentence “there is a hen in the yard”. Now imagine that Zoe does not know whether there is a hen in the yard. For example, it might be the case that she has no access to information about whether there is or is not a hen in the yard. In this case her lack of information means that she will consider two scenarios as being possible, one in which there is a hen in the yard and one in which there is not.