In Doctor Strange, the Ancient One knows that she is going to die soon because she cannot look past a point in the future, and believes it to be when she will die.
However, we also know from Infinity War, that Doctor Strange was able to look past the point of his own death, and determine how to undo the “snap”, but we can put that down to the assistance of the eye of Agamatto and the Time Stone.
However, the question remains: Why is it that you can’t look into the future past your own death?
There are a couple of possibilities.
The Ancient One didn’t make use of the Time Stone in the same way as Doctor Strange. Whether she couldn’t or wouldn’t is open to interpretation, but Strange’s particular use of the stone did give him the ability to see past his own death.
Perhaps one needs to be alive in a given time in order to see into it, and there were no futures after The Ancient One’s death in which she returned to life, whereas for Strange there were. She couldn’t see that future because she wasn’t alive in it. This does raise the possibility that there were other futures than the six million he mentioned that resulted in Thanos’ defeat… but that Strange couldn’t see those because he wasn’t alive in any of them.
However, Strange was also dead during the time of Thanos’ defeat, so if that’s the case, it seems illogical that he would be able to see into it. Unless being able to revive after the fact was evidence that some sort of defeat happened, allowing him to come back in the first place.
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