I just watched the second episode of the second series where the Doctor goes back to 1879 and sorts out a problem (won't spoil it for those who are yet to see it). But the episode reminded me of a question that's been floating around for a while - If the Doctor is fighting aliens or whatever who are trying to take over/destroy the world IN THE PAST, why do we (and his sidekick Rose) live in a world where the invasion/destruction never occured? We can assume that until the Doctor went back and intervened, the aliens/monsters/daleks/whatever were successful in their plans and it was only a visit by the doctor that, to quote another great sci-fi/time travel show, 'put right what once went wrong'. Are we living in a world where the doctor has already fixed/thwarted these things and therefore they never happened as far as we are concerned? Or is he visisting alternate timelines? Or is it merely something the writers didn't think of and we should just ignore?