  The present essay provides a new metaphysical interpretation of Relational
Quantum Mechanics (RQM) in terms of mereological bundle theory. The essential
idea is to claim that a physical system in RQM can be defined as a mereological
fusion of properties whose values may vary for different observers. Abandoning
the Aristotelian tradition centered on the notion of substance, I claim that
RQM embraces an ontology of properties that finds its roots in the heritage of
David Hume. To this regard, defining what kind of concrete physical objects
populate the world according to RQM, I argue that this theoretical framework
can be made compatible with (i) a property-oriented ontology, in which the
notion of object can be easily defined, and (ii) moderate structural realism, a
philosophical position where relations and relata are both fundamental.
Finally, I conclude that under this reading relational quantum mechanics should
be included among the full-fledged realist interpretations of quantum theory.
