  Of what are experimental quantum propositions primary bearers? As it is
widely accepted in the modern literature, rather than being bearers of truth
and falsity, these entities are bearers of probability values. Consequently,
their truth values can be regarded as no more than degenerate probabilities
(i.e., ones that have only the values 0 and 1). The mathematical motivation for
precedence of probabilistic semantics over propositional semantic for the logic
of experimental quantum propositions is Gleason's theorem. It proves that the
theory of probability measures on closed linear subspaces of a Hilbert space
(which represent experimental quantum propositions) does not admit any
probability measure having only the values 0 and 1. -- By contrast, in the
present paper, it is proclaimed that experimental propositions about quantum
systems are primary bearers of truth values. As this paper demonstrates,
algebraic properties of separable Hilbert spaces of finite dimension equal or
greater than 2 do not allow in valuations (that is, truth assignments) which
are dispersion-free, i.e., total functions from the set of atomic propositions
to the set of two objects, true and false. Providing a probability function can
be interpreted as a measure of the (un)certainty in the assignment of truth
values, the fact that valuations cannot be dispersion-free gives rise to
probabilistic semantics for the logic of experimental quantum propositions.
