Trousers of Time:
Every choice you make at a crossroad branches off to another fate. On Discworld, the Trousers of Time are hung on the peg of What Might Have Been.
Species embarrassment:
When you’re ashamed to be swimming in the same gene pool as some people. See: Hogfather (1996).
Mercy and justice:
“The good are innocent and create justice. The bad are guilty, which is why they invent mercy.” — Witches Abroad (1991)
Headology:
Why shatter an illusion when you can find a use for it. “A psychiatrist, dealing with a man who fears he is being followed by a huge and terrible monster, will endeavour to convince him that monsters don’t exist. Granny Weatherwax would simply give him a chair to stand on and a very heavy stick.” — Maskerade (1995)
Deaf musicians:
“Deafness doesn’t prevent composers hearing the music. It prevents them hearing the distractions.” — Soul Music (1994)
