Since I read and review Terry Pratchett's books fairly often, I figured I'd mix this one up a bit and just mention two quotes that really struck me and indicate his incredibly sharp intelligence.
"'Come to gloat?' whispered Rincewind. Death shrugged.
'I HAVE COME TO SEE THE FUTURE,' he said.
'This is the future?'
'A FUTURE,' said Death.
'It's horrible,' said Rincewind.
'I'M INCLINED TO AGREE,' said Death.
'I would have thought you'd be all for it!'
'NOT LIKE THIS. THE DEATH OF THE WARRIOR OR THE OLD MAN OR THE LITTLE CHILD, THIS I UNDERSTAND, AND I TAKE AWAY THE PAIN AND END THE SUFFERING. I DO NOT UNDERSTAND THIS DEATH-OF-THE-MIND.'"
There are two salient points in this quote: one inadvertently references Pratchett's own future; wracked by Alzheimer's, he chose his own way out and made his own death to end his suffering. Clearly this is something he felt strongly about. The second is this "DEATH-OF-THE-MIND." In this scene, our heroes come upon a crowd of people being seduced into singlemindedness by a charismatic speaker. Nevermind that he's crazy, that what he says makes no sense; people are eating it up. We see this all the time, the brainwashed masses fixing their wide eyes and simple minds on the easiest, most charismatic speakers and ideas. It's easier than thinking for oneself.
The other quote: "...space is not really big, it is simply somewhere to be big in." I love this, because it's so true and so simple and such a clever way of describing what is so difficult to describe. What a gift Sir Terry was to the world.