Crikey! With few clues this wasn't easy to find. However, I like a challenge, so ...
Rincewind agreed moodily. He tried to explain that magic has indeed once been wild and lawless, but had been tamed back in the mists of time by the Olden Ones, who had bound it to obey among other things the Law of Conservation of Reality; this demanded that the effort needed to achieve a goal should be the same regardless of the means used. In practical terms this meant that, say, creating the illusion of a glass of wine was relatively easy, since it involved merely the subtle shifting of light patterns. On the other hand, lifting a genuine wineglass a few feet in the air by sheer mental energy required several hours of systematic preparation if the wizard wished to prevent the simple principle of leverage flicking his brain out through his ears.
Colour of Magic. Page 79.
I really hope that this is the passage that is wanted. CoM was the 3rd book I trawled through to find it!
Going to my school was an education in itself. Which is not to be confused with actually getting an education (Schultz)