Despite having traveled aboard the TARDIS for many, many years, the Doctor still isn't sure how to pilot her. It seems the sentient, spacetime machine doesn't take any course the Doctor sets as a direct order but rather as a suggestion. If the Doctor wants to go to France in 1699, for instance, but some historic event is happening in 17th century Germany, the TARDIS will go there instead. Close enough, right?
As a result, the Doctor often ends up arriving at a place just when history is at its tipping point. The question then becomes: is the Doctor fixing history to the way it should be, or is history the way it is because he intervened? This chicken-or-the-egg question plagues all time travelers. Let's take a look at some of those instances.