Infinite temporal flux

The infinite temporal flux related to the flexibility of changing history, meaning time was always in flux and protecting the past was vital to preserving the future, as opposed to fixed points in time.

According to the Tenth Doctor, the mechanics of infinite temporal flux worked much like the basic conceit of the film Back to the Future: Marty McFly began to fade out of existence after changing his family's personal history. When Martha Jones questioned the human race ending in 1599 when she was from the 21st century, the Doctor explained the principle, after which she worried about fading away. (TV: The Shakespeare Code) When the New Dalek Paradigm destroyed humanity in 1963, 2010-native Amy Pond began to fade, requiring the Eleventh Doctor to use a Chronon Blocker to stave off the fading temporarily. (GAME: City of the Daleks)

Similarly, although she was unaffected, Sarah Jane Smith stated Earth wasn't destroyed by Sutekh in 1911, but the Fourth Doctor showed her an alternate, devastated 1980 and explained "every point in time has it's alternative." (TV: Pyramids of Mars)