DOCTOR: History sometimes gives us a terrible shock, and that is because we don’t quite fully understand. Why should we? After all, we’re too small to realize its final pattern. Therefore don’t try and judge it from where you stand.

DOCTOR: Even after all this time he cannot understand. I dare not change the course of history. Well, at least I taught him to take some precautions. He did remember to look at the scanner before he opened the doors. Now they're all gone. All gone. None of them could understand. Not even my little Susan, or Vicki. And as for Barbara and Chatterton. Chesterton. They were all too impatient to get back to their own time. And now, Steven. Perhaps I should go home, back to my own planet. But I can't. I can't.