"So the Primer was correct on that point. Now, as to the fact that 
killing people is a more complicated business in practice than in theory, I 
will certainly concede your point. But I think it is not likely to be the only 
instance in which real life turns out to be more complicated than what you have 
seen in the book. This is the Lesson of the Screwdriver, and you would do well 
to remember it. All it amounts to is that you must be ready to learn from 
sources other than your magic book."