Ah, I think the old favourites deserve an outing every now and then...
Sir Humphrey: Polaris is a ramshackle old system! The Soviets might easily develop a multi-layered ballistic missile defence system which could intercept Polaris.
Jim: By when?
Sir Humphrey: Well, in strategic terms, any day now.
Jim: By what year, precisely?
Sir Humphrey: 2020, but that's sooner than you think.
Jim: And are you saying that this nuclear defence system would stop all 192 Polaris missiles?
Sir Humphrey: Well no, not all. Virtually all. 97%.
Jim: So that would leave about five bombs that would get through?
Sir Humphrey: Precisely, a mere five.
Jim: Enough to obliterate Moscow, Leningrad, Minsk ...
Sir Humphrey: Yes, but that's about all!
...
Jim: It's a bluff, I probably wouldn't use it.
Sir Humphrey: Yes but they don't know that you probably wouldn't.
Jim: They probably do.
Sir Humphrey: Yes, they probably know you probably wouldn't but they can't be certain.
Jim: They probably, certainly know that I probably wouldn't.
Sir Humphrey: Yes, but even though they're probably certain you know you probably wouldn't they don't certainly know that although you probably wouldn't, there is no probability that you certainly would.
Jim: What?
...
Sir Humphrey: Bernard, what is the purpose of our defence policy?
Bernard: To defend Britain?
Sir Humphrey: No Bernard, it is to make the people think that Britain is defended.
Bernard: The Russians?
Sir Humphrey Not the Russians, the British! The Russians know it's not.