  The action of the discrete symmetries on the scalar mode functions of the de
Sitter spacetime is studied. The invariance with respect to a combination of
discrete symmetries is put forward as a criterion to select a certain vacuum
out of a family of vacua. The implications of the choices for eigenfunctions of
various common sets of commuting operators are explored, and the results are
compared to the different original choices from the literature that have been
utilized in order to exhibit thermal effects.
