  Motivated by considerations in the foundations of quantum mechanics and
inspired by the literature on vague predicates, we introduce the concept of an
opaque predicate. While in the case of vague predicates there is a kind of
indeterminacy with respect to the predicate, in the sense that the vagueness
concerns whether a well-determined object satisfies it or not, in the case of
opaque predicates the indeterminacy is with regard to the objects which should
satisfy them. In other words, their extensions are not well-defined, despite
the fact that the conditions for an object to satisfy the predicates are
well-known. We suggest that such opaque predicates (and more generally, what we
call opaque relations) can be characterized by a logic which encompasses a
semantics founded in quasi-set theory, and call their extensions veiled sets.
