Exactly Lillith (I lurve these types of discussions) but if someone foresaw a future event and did something to prevent it happening, what did they see?

They originally saw the future but changed the present so that particular future couldn't happen, so then how did they see it if, looking back from the future event, they had prevented it ever happening.

It's the classical metaphysical paradox.
lets say for example that you had a working time machine.
You can't travel to the future because it hasn't happened yet so you can only travel to the past, But, from the point of view of people in the past, you are coming from the future, which hasn't happened yet, so it isn't possible

This type of discussion really needs alcahol to get off the ground, have you ever noticed that after a good drinking session, stuff like this makes perfect sense and you can quite easilly solve all the problems in the universe but can't remember how you did it the next morning