Any adequate theory of agency must be able to describe explicit knowledge correctly, because this is the concept of knowledge that can provide justification and explanation for action. What an agent chooses to do depends on his explicit, and not on his implicit knowledge. As I have argued in the last chapter, any theory of explicit knowledge must avoid logical omniscience. Because modal epistemic logic cannot characterize explicit knowledge, a number of alternative approaches have been proposed.
In this chapter I shall examine some of the more prominent attempts to develop logics for explicit knowledge of non-omniscient agents. I shall show that although the existing approaches to explicit knowledge can solve the logical omniscience problem, they are not suited to characterizing the information states of agents. In order to be useful, a logic of explicit knowledge must satisfy some additional conditions besides the lack of omniscience. This explains why all influential agent theories use modal epistemic logics for formalizing the informational aspect of agents, although the modal approach does not capture the explicit notion of knowledge adequately.