  The status of locality in quantum mechanics is analyzed from a nonstandard
point of view. It is assumed that quantum states are relative, they depend on
and are defined with respect to some bigger physical system which contains the
former system as a subsystem. Hence, the bigger system acts as a reference
system. It is shown that quantum mechanics can be reformulated in accordance
with this new physical assumption. There is an important consequence of this
dependence: states may not be comparable, i.e., they cannot be checked by
suitable measurements simultaneously. This special circumstance is fully
reflected mathematically by the theory. Especially, it is shown that certain
joint probabilities (or the corresponding combined events) which play a vital
role in any proof of Bell's theorem do not exist. The conclusion is that the
principle of locality is fully valid in quantum mechanics, and one has to give
up instead of locality an intuitively natural-looking feature of realism,
namely, the comparability of existing states.
