Fixed point in time
A fixed point in time is an event or circumstance that must occur as history has perceived it to occur in order to allow the smooth and uninterrupted flow of historical events.
While it is generally impossible to undo a fixed point due to the damage it would cause to spacetime, since it is a perceived event, there is some "play" or "slack" available to manipulate events, whereby the underlying circumstance can be modified to a different outcome as long as the overall historical tapestry does not notice the difference.
Example:
Original event: Person X is assassinated in broad daylight starting a World War.
Modification: Person X who was shot is actually a clone of the real person X implanted by Time travellers.
Result: History carries on as normal, but the real Person X is saved.
Time Lords, as a nature of their very being, perceive all alternative versions of a moment at once, though all timelines and permutations. They usually filter out this information at a conscious level, but this enormously intimate connection with the fabric of realities enables them to perceive the relative significance of a given timeline or course of events, which allows them to know which ones can be easily altered and which must be left alone in order to maintain the structural integrity of the fabric of spacetime.
Lesser races, not possessing the perceptual capacities of Time Lords, often accuse them of being arbitrary in their designation of which events are fixed and which are not, mainly due to their discomfort with the implications of the responses they are given.]
To give an example of how troublesome these calculations can be, consider:
At one point the Time Lord known as The Doctor lost two of his friends, Amy and Rory, because after their last interaction with the Weeping Angels, he determined even returning to New York to pick them back up would cause a paradox so terrible that New York would rip itself apart.
Other non-Time Lord observers immediately (and reasonably, given their understanding of things) wondered after the fact, why couldn’t the Doctor just go pick up The Ponds in New Jersey, Connecticut, Washington, etc. somewhere outside of New York?
The problem was, as the Doctor knew, New York would still burn. He could not interfere. Under normal circumstances he might have gone back and made subtle changes to try and preserve the apparent flow of history such as erecting false gravestones, etc, but he knew there was so much temporal scar tissue, and the number of paradoxes that had already been inflicted on that nexus of timelines, that it would have ripped apart if he had tried to do one more thing. He had to leave it alone. Normally he could perform some surgery, this time too much surgery had already been performed.
