 The Kochen-Specker theorem (1967) is an obstacle to any traditional realist interpretation of quantum mechanics, since it proves the impossibility of ascribing precise values to all physical quantities (observables) of a quantum system simultaneously, while preserving the functional relations between commuting observables. (See the entry on the  the Kochen-Specker theorem.)  Therefore, realist non-collapse interpretations are committed to selecting a (Boolean) subset of definite-valued observables out of all observables. Each modal interpretation thus supplies a “rule of definite-value ascription” or “actualization rule”, which picks out, from the set of all the observables of a quantum system, the subset of definite-valued properties, which may change in time.