K9: Optimism. Belief that everything will work out well. Irrational, bordering on insane.

DRAX: Hello, Theet. How you been, boy?

DOCTOR: What?

DRAX: It is Theet, innit? Theta Sigma? Yeah, 'course it is. Remember me, ay?

ROMANA: I see. So where are we going?

DOCTOR: Going? I don’t know.

ROMANA: You have absolutely no sense of responsibility whatsoever.

DOCTOR: What?

(Romana backs him up against a wall, poking him with every word)

ROMANA: You’re capricious, arrogant, self-opinionated, irrational, and you don’t even know where we’re going.

DOCTOR: Think positive. Why do you always assume the worst?

ROMANA: Because it usually happens.

DOCTOR: Empirical poppycock. Where’s your joy in life? Where’s your optimism?

ROMANA: It opted out.