 Especially problematical from an orthodox perspective is quantum cosmology, for which the relevant quantum system is the entire universe, and hence there is no observer outside the system to cause collapse of the wave function upon measurement. In this setting Bohmian models have clarified the issue of the inevitability of the presence of singularities in theories of quantum gravity (Falciano, Pinto-Neto, &amp; Struyve 2015). For a discussion of how a Bohmian perspective can eliminate some conceptual difficulties arising in quantum gravity, such as the problem of time and the problem of the absence of sufficiently many diffeomorphism-invariant observables, see Goldstein &amp; Teufel 2001. 