  Epistemic logic has become a major field of philosophical logic ever since
the groundbreaking work by Hintikka (1962). Despite its various successful
applications in theoretical computer science, AI, and game theory, the
technical development of the field has been mainly focusing on the
propositional part, i.e., the propositional modal logics of "knowing that".
However, knowledge is expressed in everyday life by using various other
locutions such as "knowing whether", "knowing what", "knowing how" and so on
(knowing-wh hereafter). Such knowledge expressions are better captured in
quantified epistemic logic, as was already discussed by Hintikka (1962) and his
sequel works at length. This paper aims to draw the attention back again to
such a fascinating but largely neglected topic. We first survey what Hintikka
and others did in the literature of quantified epistemic logic, and then
advocate a new quantifier-free approach to study the epistemic logics of
knowing-wh, which we believe can balance expressivity and complexity, and
capture the essential reasoning patterns about knowing-wh. We survey our recent
line of work on the epistemic logics of "knowing whether", "knowing what" and
"knowing how" to demonstrate the use of this new approach.
