 There is room, moreover, for yet more skepticism here. The arguments are prima facie strong that the analogue of Hawking radiation should manifest in a wide range of systems, as a purely kinematical effect following directly from a few simple kinematical principles that all those systems satisfy (Unruh 2014). Nonetheless, true gravitational black holes are radically different from all the proposed analogue systems, in a variety of extensive and deep ways, as is general relativity as a physical theory from all the theories governing those other types of systems. As the debate and dissension discussed in  section 6.1  illustrates, the fundamental physics of Hawking radiation may not be well enough understood to have confidence that some confounding physical factor cannot be present in purely gravitational systems that is not present in any of the analogue systems, a factor that would block production of Hawking radiation by true black holes. In other words, there seems prima facie little reason to have faith that the universality condition holds, except on the basis of purely theoretical arguments pertaining to systems we have no empirical experience of nor access to whatsoever. 