...the vacuum is the state in which no particles exist. It carries no physical properties; it is structureless and uniform. I emphasise this by saying that the vacuum is not only the state of minimum energy, it is the state of ZERO energy, ZERO momentum, ZERO angular momentum, ZERO charge, ZERO whatever.
Quite adamant, isn't he? Carl Brannen places much importance on Schwinger's ideas in his derivation of the lepton masses, in particular the measurement algebra. This is conceptually important, because it stresses experiment and the link between input and output states, rather than the usual arbitrary independence of these. Observe that this is more than a mere recognition of the interdependence of states, because it sees more reality in the measurement itself than in intermediate states.