  Laws of motion given in terms of differential equations can not always be
derived from an action principle, at least not without introducing auxiliary
variables. By allowing auxiliary variables, e.g. in the form of Lagrange
multipliers, an action is immediately obtained. Here, we consider some ways how
this can be done, drawing examples from the literature, and apply this to
Bohmian mechanics. We also discuss the possible metaphysical status of these
auxiliary variables. A particularly interesting approach brings the theory in
the form of a gauge theory, with the auxiliary variables as gauge degrees of
freedom.
