  A composite quantum system has properties that are incompatible with every
property of its parts. The existence of such global properties incompatible
with all local properties constitutes what I call "mereological holism"--the
distinctive holism of Quantum Theory. Mereological holism has the dramatic
conceptual consequence of making untenable the usual understanding of the
"quantum system" as being a "physical object", since composed objects have
properties compatible with those of its parts. The notion of "property" can be
extended in a unique way to the whole class of operational probabilistic
theories (shortly OPTs), of which the most relevant cases are Quantum Theory
and Classical Theory. Whereas Classical Theory is not mereologically holistic,
we can now search for other OPTs that are so. Within the OPT framework the role
of the "system" is that of an input-output connection between two objective
events. In non holistic theories, such as Classical Theory, the system can
still be regarded as an "object". On the contrary, in holistic theories
interpreting "system" as "object" constitutes an hypostatization of a
theoretical notion.
