  We consider the situation of a physical entity that is the compound entity
consisting of two 'separated' quantum entities. In earlier work it has been
proven by one of the authors that such a physical entity cannot be described by
standard quantum mechanics. More precisely, it was shown that two of the axioms
of traditional quantum axiomatics are at the origin of the impossibility for
standard quantum mechanics to describe this type of compound entity. One of
these axioms is equivalent with the superposition principle, which means that
separated quantum entities put the linearity of quantum mechanics at stake. We
analyze the conceptual steps that are involved in this proof, and expose the
necessary material of quantum axiomatics to be able to understand the argument
