Time can be changed, history cannot. Events are only history if an observer already knows they will happen. So for example, the Doctor knows who Napoleon is upon meeting him and what he will do, so he kills him. This would be breaking a fixed point and cause a paradox. If the Doctor had a companion from Ancient Rome and brought him to meet Napoleon and the companion kills Napoleon for being a dick, not knowing Napoleon's future actions it would not be history to them and time will change. The wrinkle is even if the companion killed him the Doctor would still be there knowing the change and will still be a fixed point. If they then went to some future alien planet and killed a guy who would become a dictator in the current Web of Time, then that's perfectly fine and crystallizes as history. It's a concept related to quantum science such as observation setting particle spin and the double-slit experiment.
Dominic D
2020-05-11 23:21:51 +0000 UTC
Barbara mentions in the Aztecs that a major reason she was trying to stop the sacrifices was the vain hope that Cortez and the Spaniards would look upon them more humanely after contact.