 Once we have established that we are interested in maximal spacetimes, the next issue is what sort of path incompleteness is relevant for singularities. Here we find a good deal of controversy. Criteria of incompleteness typically look at how some parameter naturally associated with the path (such as its proper length) grows. One generally also places further restrictions on the paths that one considers—for example, one may rule out paths that could be traversed only by particles undergoing unbounded acceleration in a finite period of time. A spacetime, then, is said to be singular if it possesses a path such that the specified parameter associated with that path cannot increase without bound as one traverses the entirety of the maximally extended path. The idea is that the parameter at issue will serve as a marker for some manifestly physical property, such as the time experienced by a particle or observer, and so, if the value of that parameter remains finite along the whole path, then we have run out of path in a finite amount of time, as it were. We have hit an edge or a “tear” in spacetime. 