system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] what determines if a point in time is fixed or not?
gpt
Fixed Points in time are enormous, tumultuous events that set in motion the broadest strokes of history, and causality itself depends on them to function.  They are, oddly enough, an entirely natural phenomenon.

While the fine details surrounding such an event can change, and often do, the outcome will *always* remain the same, even if an attempt to change history is made -- we see this in The Waters of Mars.