The Doctor, Romana I and others in “The Armageddon Factor”

What's the matter?

Look! Radiation levels you wouldn't believe!

Good heavens! You could fry eggs in the street.

But that means...

What?

There must be a huge nuclear war going on down there!

None at all, no.

Well what else could it be?

I don't know. Probably someone throwing a huge breakfast party. Think po- Why do you always assume the worst?

Because it usually happens.

Empirical poppycock! Where's your joy in life? Where's your optimism?

It opted out.

Optimism: Belief that everything will work out well. Irrational, bordering on insane.

Oh, do shut up, K9. Listen, Romana, Whenever you go into a new situation, you must always believe the best until you find out exactly what the situation's all about. Then, believe the worst.

Ah, but what happens if it turns out not to be the worst after all?

Don't be ridiculous. It always is.