From the intro to an advanced math book:
5. The book contains an abundance of homework problems. They are not
graded by level of difficulty since life does not present problems that way.
Devil’s Advocate: What’s not to like, Dave? He’s right, isn’t he? Life does not present problems that way.
SD: He’s right if we are talking about homework that prepares one for life.
But there is another kind of homework, namely,
homework that aids in mastering the material.
And new material is best mastered by going from the easier to the harder.
First we learn to crawl, etc.
DA: I’m glad you pointed this out. My one year old son had started to crawl all over the house. I was having none of that. I took the little fellow out into the street, with traffic constantly streaming by, and forced him to stand. He started crying.
“Son”, I told him, “I can’t let you crawl freely in a protected environment. Life is not like that. Get up and walk, son, and watch out for those cars.”
SD: And what happened?
DA: He failed his math course.