Pratchett Vimes quote?

Help. I can’t recall which book and how it went! It was Vimes.

Something about what kind of man has a policeman inside himself to keep the darkness out and the answer that the policeman was there to keep the darkness in.

Thanks.

This is from “Thud!,” I think.

Sounds like “Thud!” but it could well be “Snuff”.

There’s a similar but not quite right passage in “The Fifth Elephant”, too, about the darkness inside Vimes.

The 6th quote on this “Thud!” quotes page sounds likely:

That’s it!

Thanks!