Magic, judicious control of

Not doing any magic at all was the chief task of wizards -- not "not doing any magic" because they couldn't do magic, but not doing magic when they could do and didn't. Any ignorant fool can fail to turn someone else into a frog. You have to be clever to refrain from doing it when you knew how easy it was. There were places in the world commemorating those times when wizards hadn't been quite as clever as that, and on many of them the grass would never grow again [154].

Pratchett, Terry. Going Postal. HarperCollins, 2004. ISBN 0060502932