  Paradox mongers can be reassured in that if a paradox is lost in  shifting the focus from time travel itself to time machines, then a  paradox is also gained: if it is possible to operate a time machine  device that produces CTCs, then it is possible to alter the structure of spacetime such that determinism fails; but by undercutting  determinism, the time machine undercuts the claim that it is  responsible for producing CTCs. But just as the grandfather paradox  is a crude way of making a point, so this new paradox is a crude way  of indicating that it is going to be difficult to specify what it  means to be a time machine. This is a task that calls not for paradox mongering but for scientifically informed philosophizing. The present article will provide the initial steps of this task and will indicate what remains to be done. But aside from paradoxes, the main payoff of the topic of time machines is that it provides a quick route to the  heart of a number of foundations problems in classical general  relativity theory and in attempts to produce a quantum theory of  gravity by combining general relativity and quantum mechanics. We  will indicate the shape of some of these problems here, but will  refer the interested reader elsewhere for technical details.