 There are also more technical issues that await further discussion. Modal interpretations as discussed here are based on the standard Hilbert space formalism of quantum mechanics. However, Brown, Suárez, and Bacciagaluppi (1998) argue that there is more to quantum reality than what is described by operators and quantum states: they claim that gauges and coordinate systems are important to our description of physical reality as well, while modal interpretations (AM, BDMI and SDMI) have standardly not taken such things into consideration. In a similar vein, in the context of the MHI it has been argued that Galilean space-time symmetries endow the formal skeleton of quantum mechanics with physical flesh and blood and identify the fundamental physical magnitudes that allow the theory to be applied to concrete physical situations. If the set of definite-valued observables of a system is postulated to be invariant under Galilean transformations (the intuition being that a mere change in inertial reference frame does not change this set), the MHI rule of actualization can be reformulated in an explicitly invariant form, in terms of the Casimir operators of the Galilean group (Ardenghi, Castagnino, and Lombardi 2009, Lombardi, Castagnino, and Ardenghi 2010).