  The question of whether a Thornian time machine—a device that  produces CTCs—can be seen to be at work only makes sense if the spacetime has at least three features: temporal orientability, a  definite time orientation, and a causally innocuous past. In order to make the notion of a CTC meaningful, the spacetime must be  temporally orientable (i.e., must admit a consistent time  directionality), and one of the two possible time orientations has to be singled out as giving the direction of   time.[7]  Non-temporal orientability is not really an obstacle since if a given general relativistic spacetime is not temporally orientable, a  spacetime that is everywhere locally the same as the given spacetime  and is itself temporally orientable can be obtained by passing to a  covering   spacetime.[8]  How to justify the singling out of one of the two possible  orientations as future pointing requires a solution to the problem of the direction of time, a problem which is still subject to lively  debate (see Callender 2001). But for present purposes we simply  assume that a temporal orientation has been provided. A CTC is then  (by definition) a parameterized closed spacetime curve whose tangent  is everywhere a future-pointing timelike vector. A CTC can be thought of as the world line of some possible observer whose life history is  linearly ordered in the small but not in the large: the observer has  a consistent experience of the “next moment,” and the  “next,” etc., but eventually the “next  moment” brings her back to whatever event she regards as the  starting point.