The Doctor:  Ah , Brigadier , quickly think : Did you go into the TARDIS with Tegan and Nyssa ?
Brigadier Alistair Gordon Lethbridge Stewart: Well , I ca n't remember . Does it really matter ?
The Doctor: Of course it matters ! Can you imagine what would happen if you walked out of the TARDIS in 1977 and met yourself in _ 1983 _ ?
Brigadier Alistair Gordon Lethbridge Stewart: Oh , that 's ridiculous .
The Doctor: It 's not ridiculous , Brigadier . Almost certainly catastrophic .
Brigadier Alistair Gordon Lethbridge Stewart: What do you mean ? That I could be two people ?
The Doctor: Obviously . You 'd exist twice over , and if the two of you met you 'd short out the time differential , do n't you see ? The Blinovitch Limitation Time Effect ? Oh , dear . As Tegan would say - zap !