  Epistemic logic programs constitute an extension of the stable models
semantics to deal with new constructs called subjective literals. Informally
speaking, a subjective literal allows checking whether some regular literal is
true in all stable models or in some stable model. As it can be imagined, the
associated semantics has proved to be non-trivial, as the truth of the
subjective literal may interfere with the set of stable models it is supposed
to query. As a consequence, no clear agreement has been reached and different
semantic proposals have been made in the literature. Unfortunately, comparison
among these proposals has been limited to a study of their effect on individual
examples, rather than identifying general properties to be checked. In this
paper, we propose an extension of the well-known splitting property for logic
programs to the epistemic case. To this aim, we formally define when an
arbitrary semantics satisfies the epistemic splitting property and examine some
of the consequences that can be derived from that, including its relation to
conformant planning and to epistemic constraints. Interestingly, we prove
(through counterexamples) that most of the existing proposals fail to fulfill
the epistemic splitting property, except the original semantics proposed by
Gelfond in 1991.
