Even the most precise wordsmiths have blind spots in their writing -- words they cannot spell without looking up every time, grammatical rules they can't for the life of them get right and so they'll painstakingly write around to avoid screwing it up, hyphenglycemia, apostrophobia, and so on. Often those old rules from English class really do still come in handy. Like the one that bails me out every time I face one of my weaknesses: The principle is your ple.