If we build a time machine, anyone may use it and cause the past or future to change. Even if we establish an institution for using it that checks for altering the past, we still have the problem of sensing the natural past from an altered past, because we can't know whether some John or Jane were born - if they aren't famous. So unintended changes may cause someone not to be born, but was born in the natural past.

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Moderator Note

If we build a time machine begs the question of whether we can build a time machine.

Try opening a thread not based on a logical fallacy