Paradox

Jack's forehead wrinkled with effort. "So General Hammond knew to give you the note because he'd met us in the past."

Sam nodded.

"And he wouldn't have met us in the past if he hadn't...met us in the past."

"I think that's right, sir."

"Ok, I get that," he said, and she pretended to believe him. "I still don't understand the Cassandra bit, though."

Sam suppressed a sigh. "Why not?"

"We had no grandfather clause. We should have been trapped in the future, where you couldn't tell her to help us...in the future."

"That's the paradox, sir."

"Ah."