Awareness has been shown to be a useful addition to standard epistemic logic for many applications. However, standard prepositional logics for knowledge and awareness cannot express the fact that an agent knows that there are facts of which he is unaware without there being an explicit fact that the agent knows he is unaware of. We propose a logic for reasoning about knowledge of unawareness, by extending Fagin and Halpern's Logic of General Awareness. The logic allows quantification over variables, so that there is a formula in the language that can express the fact that "an agent explicitly knows that there exists a fact of which he is unaware". Moreover, that formula can be true without the agent explicitly knowing that he is unaware of any particular formula. We provide a sound and complete axiomatization of the logic, using standard axioms from the literature to capture the quantification operator. Finally, we show that the validity problem for the logic is recursively enumerable, but not decidable.
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