PERI: Circular logic will only make you dizzy, Doctor.

DOCTOR: The most likely explanation, of course, is that I haven't synchronised properly yet. Some kind of time-slip in the subconscious.

PERI: Perhaps you should see a doctor.

DOCTOR: Are you trying to be funny?

PERI: No, it was just a suggestion.

DOCTOR: Actually, that's not such a bad idea.