My estimation is how the show (and all other shows who use any kind of time travelling machine) are able to all humans to visit a time from before they too were born, for example Doctor Who uses Pompeii as one of the destinations for Donna, a woman born in the around the 1980's time.The mere fact that it exists will allow it to travel back to before it has been built. However, a paradox would be created if a time machine were used to travel back to infact stop the very same time machine from being built as there would be no time machine to appear and allow it to stop being built.I could go on into more detail however I hope this small explanation answers your query.Matt
Aaah that makes more sense. Smart boy.
Doesn't travel back along its own timeline. And besides, it's not an object or a person or a planet. It's an entire dimension. Dimensions are excluded from the paradoxes of time travel because they are by definition separate from it. Spacetime of one dimension is separate from another. Therefore, the dimension that is the TARDIS is separate from the events--even that created it--that reside in the spacetime fabric of the dimension we know.
Wow, I feel safer now :)