Because you're an Ancient. Right? I'm sorry. [he doesn't know if he ever told Aerith that he knew. Probably not, because it had never been important to him. Not even when he learned more about it, as Sephiroth slowly learned about who he was in all of that, and slowly descended into madness]

He took me and Cloud after what happened in Nibelheim. He wanted to try and turn one of us into the next Sephiroth.