 One of the most common arguments that incomplete paths and non-maximal spacetimes are physically unacceptable, and perhaps the most interesting one, coming as it does from physicists rather than from philosophers, invokes something very like the Principle of Sufficient Reason: if whatever creative force responsible for spacetime could have continued on to create more of it, what possible reason could there have been for it to have stopped at any particular point (Penrose 1969; Geroch  1970)?[8]  An opponent of this view could respond that it implicitly relies on a certain picture of physics that may not sit comfortably with general relativity, that of the dynamical evolution of a system. An advocate of this viewpoint would argue that, from a point of view natural for general relativity, spacetime does not evolve at all. It just sits there, once and for all, as it were, a so-called block universe (Putnam 1967; the entries  Time Machines,  Time Travel and  Being and Becoming in Modern Physics).  If it happens to sit there non-maximally, well, so be it. This kind of response, however, has problems of its own, such as with the representation of our subjective experience, which seems inextricably tied up with ideas of evolution and change. Those sorts of problem, however, do not seem peculiar to this dispute, but arise from the character of general relativity itself: “dynamical evolution” and “time” are subtle and problematic concepts in the theory no matter what viewpoint one takes (Stein 1968, 1970, 1991). 