My proposal: the ideas that goodness or evil are substances and they can formed into magic objects such as sword made of pure evil.

Of course, some novels also subvert this delightfully. Patricia Wrede's The Seven Towers, for instance, is all about exactly what goes wrong when you try to make a magical object out of pure good.

(Edit: that is, Wrede does not literally spend the whole book talking about this problem. It is merely mentioned as backstory. But still.)