Sanderson's Second Law

A few years back, I wrote an essay on creating magic systems that I titled Sanderson's First Law. It had to do with the nature of foreshadowing as it relates to solving problems with magic. In that essay, I implied that I had other "laws" for magic systems that I'd someday talk about. Well, that time has come, as I've finally distilled my thoughts for the second law into an explanation that will work.