Fringe, Season 3

Peter:
[appearing to Olivia in a vision] You know why you didn't die today, right? Because you didn't know the protocol. If you had stopped for oxygen, you'd be dead right now, but you did something that he couldn't factor in. You kept running. You know why you did that. It's the same reason why you thought you saw Walter in the hospital. It's the same reason you think you see me now. You're not from this world, Olivia. You're not her.

Olivia:
You're not real.

Peter:
[smiles] Real is just a matter of perception. I am here, and I'm a part of you that you have to hold on to. You can't forget who you are, Olivia. You can't forget where you're from. You can't forget this. [kisses her softly]