  It is well known that the dynamical mechanism of decoherence may cause
apparent superselection rules, like that of molecular chirality. These
`environment-induced' or `soft' superselection rules may be contrasted with
`hard' superselection rules, like that of electric charge, whose existence is
usually rigorously demonstrated by means of certain symmetry principles. We
address the question of whether this distinction between `hard' and `soft' is
well founded and argue that, despite first appearance, it might not be. For
this we give a detailed and somewhat pedagogical exposition of the basic
structural properties of the spaces of states and observables in order to
establish a fairly precise notion of superselection rules. We then discuss two
examples: the Bargmann superselection rule for overall mass in ordinary quantum
mechanics, and the superselection rule for charge in quantum electrodynamics.
