  In 1991, Michael Gelfond introduced the language of epistemic specifications.
The goal was to develop tools for modeling problems that require some form of
meta-reasoning, that is, reasoning over multiple possible worlds. Despite their
relevance to knowledge representation, epistemic specifications have received
relatively little attention so far. In this paper, we revisit the formalism of
epistemic specification. We offer a new definition of the formalism, propose
several semantics (one of which, under syntactic restrictions we assume, turns
out to be equivalent to the original semantics by Gelfond), derive some
complexity results and, finally, show the effectiveness of the formalism for
modeling problems requiring meta-reasoning considered recently by Faber and
Woltran. All these results show that epistemic specifications deserve much more
attention that has been afforded to them so far.
