We argue that none of the existing epistemic logics can adequately serve the needs of agent theories. We suggest a new concept of knowledge which generalizes both implicit and explicit knowledge and argue that this is the notion we need to formalize agents in Distributed Artificial Intelligence. A logic of the new concept is developed which is formally and practically adequate in the following sense: first, it does not suffer from any kind of logical omniscience. Second, it can account for the intuition that agents are rational, though not hyper-rational. Third, it is expressive enough. The advantages of the new logic over other formalisms is demonstrated by showing that none of the existing systems can fulfill all these requirements simultaneously.