The Doctor thought back, but suddenly that wave hit him again and he leaned forward. He was out for a walk in the TARDIS---but those memories weren't right, somehow. They were what was making him sick? Maybe?

"I think that's what it is," he said. "I think we did something or opened something together and we've forgotten it. Every time my mind tries to make up something new, it hurts."

Through the pain, he looked over to Jack. Always known him. He couldn't understand the sentiment, but he didn't think he remembered as much as Jack did. Jack's touch wasn't familiar to him.

"Focus your mind," he said. "Try to remember. I think of the two of us, you would be the one who could."