"One man's retreating looks like a vanishing to the observers, or some of 'em anyway," Jack replies. "And that sounds about right: If you aren't getting out to stretch your legs and your boundaries, you're bound to stunt your own growth." He wonders now if this is exactly why The Doctor, his Doctor, went travelling as far through time and space as he has. He's not questioning The Doctor, he's too fond of the Time Lord, but he's starting to wonder about Gallifrey and its inhabitant.