We mustn't change history,' Susan pouted. Ian had been so awed by her work on the portal, he'd almost forgotten she was just in her teens. 'But Susan,' said the Doctor in his softest voice, 'we change history every time we step out of the door of the Ship.' 'You said that—' began Barbara. The Doctor whirled on her. 'Do you really want to know, Miss Wright?' he said. 'Do you really want to know? The TARDIS is built specifically to change history. We can visit, we can observe, and the Ship can disguise itself so no one need ever know we were there. But only so long as we never step outside. We watch it all on the scanner. My people, you see...' He paused, searching for the words. 'Doctor?' Barbara prompted. 'I couldn't do that, could you?' he said. 'It's not travel, it wouldn't be real. We've seen the most incredible things, but without stepping out of those doors, I might as well have stayed in your time, content with your television sets.