Yes, Minister, Series One (1986)

Sir Humphrey:
With Trident we could obliterate the whole of Eastern Europe.

Hacker:
I don't want to obliterate the whole of Eastern Europe.

Sir Humphrey:
But it's a deterrent.

Hacker:
It's a bluff. I probably wouldn't use it.

Sir Humphrey:
Yes, but they don't know that you probably wouldn't.

Hacker:
They probably do.

Sir Humphrey:
Yes, they probably know that you probably wouldn't. But they can't certainly know.

Hacker:
They probably certainly know that I probably wouldn't.

Sir Humphrey:
Yes, but even though they probably certainly know that you probably wouldn't, they don't certainly know that although you probably wouldn't, there is no probability that you certainly would!