Even though we don't know everything time travelers would need to know, logic would remain the same for them, I think. Go back in time, change something you didn't like. You've, just destroyed your reason for going into the past, so you wouldn't go. But that also means you couldn't have changed the past. Which brings us back to the beginning of the same circle again! If time travel really were possible, it still appears to be impossible to change the past. Even the mere presence of a time traveler from the future would, it seems, begin to change things and so disrupt the time line to the future, including the part about that particular time trip to the past. Ross