In my humble opinion, something which must have a
practical consequence. Otherwise it leads into a
maze of
endless speculation which is
tricky to
escape. By this
criterion many seemingly
insoluble problems can be
happily discarded. For
example, the problem of
free will and
determinism, when practically approached, shows that one must
act, must make a decision. Whether or not the decision is an illusion is
irrelevant to any who have felt the
agony of decision. Therefore the question becomes moot.