Fringe, Season 3

Walter:
This time you have to make different choice. And if something goes wrong, Olivia would be available to...

Peter:
Walter; Stop [hold back his tears] Olivia is dead.

Walter:
But she won't be. Not there.

Peter:
The machine? I turned that on 15 years ago.

Walter:
And all the time I sat in prison, I couldn't figure out where it came from. I knew the pieces were buried millions of years ago, but how did they get there? So deep in the past. But now I understand. I sent them there. The hole in central park. I sent them back through time. Peter, you can stop the destruction before it occurs.

Peter:
If that's the case, just don't send the machine back. They will never discover it and I'll never destroy the other universe.

Walter:
No no no. Dosen't work that way. I have already done it. And I have no choice but to do it again.

Peter:
Walter that doesn't make any sense.

Walter:
It does. It is a paradox. I can't change what happens because it's already happened. But you can make a different choice within what happened. I simply need to find a way to bring your consciousness forward to now, so that you can witness what will happen if you make the same choice. Peter, for all I know, it can be happening already. Don't you see, we can fix everything. We can cheat the rules of time.

Peter:
Imagine the repercussions.

Walter:
It can't be worst than this. It can't be worst than this.