First Thoughts are the everyday thoughts. Everyone has those. Second Thoughts are the thoughts you think about the way you think. People who enjoy thinking have those. Third Thoughts are thoughts that watch the world and think all by themselves. They’re rare, and often troublesome. Listening to them is part of witchcraft.

The people who guard the rainbow don't like those who get in the way of the sun.

Of course, it is very important to be sober when you take an exam. Many worthwhile careers in the street-cleansing, fruit-picking and subway-guitar-playing industries have been founded on a lack of understanding of this simple fact.

In an instant he became aware that the tourist was about to try his own peculiar brand of linguistics, which meant that he would speak loudly and slowly in his own language.

I could be wrong. Not being certain is what being a philosopher is all about.

The Captain of the Watch says if you're still in the City by sunrise he will personally have you buried alive.

You can't map a sense of humor. Anyway, what is a fantasy map but a space beyond which There Be Dragons? On the Discworld we know that There Be Dragons Everywhere. They might not all have scales and forked tongues, but they Be Here all right, grinning and jostling and trying to sell you souvenirs.