Thinking about a couple of recent Language Log posts about grammar checkers, passive sentences, and grammatical Cupertinos, it occurred to me that grammar checkers are a bit like Rain Man. For the benefit of younger readers (no one alive in 1988 could have missed the hype surrounding this movie), Dustin Hoffman plays Raymond (“Rain Man”), an autistic man who is also a mathematical savant. Tom Cruise is his hotshot younger brother who finds himself with the difficult task of caring for Raymond, whose inability to relate to the real world leads them into bizarre situations. Tom takes them to Las Vegas, where Raymond’s uncanny mathematical aptitude makes them big winners until casino security accuses them of cheating.
What does this have to do with grammar checkers? A computer grammar checker is like a grammatical Rain Man—a savant with an encyclopedic knowledge of grammar rules, but no common sense understanding of the real world, which sometimes leads to strange behavior. Take the famous example “Time flies like an arrow”. This sentence is notable because it has a lot of possible but quite strange interpretations, in addition to its straightforward meaning of “time moves fast the way an arrow moves fast”. It could mean, for example, something like: whenever you encounter a fly, time it in the same way you would time an arrow (using a stopwatch, perhaps). We humans naturally disregard such interpretations (they may never even occur to us). But our grammatical Rain Man sees every such possibility at the same time, and has to decide among them. And it doesn’t have our common sense to help it. Instead, it has to figure out the intended meaning of sentence by purely computational means, basically by reasoning only about things that can be counted. Such as, say, the number of times it has seen “time” used as an imperative verb vs. the number of times it’s seen it used as a noun subject, and so on.
Most of the time this all works as advertized, but it can occasionally lead to some odd decisions. Once in a while the grammar checker will make a suggestion that just looks weird, so if you blindly accept all its recommendations, you end up with those cupertinos. Letting Rain Man call the shots may sound like an easy way to get rich, but like Tom at the casino, if you’re not careful you can end up in trouble.
So should you take advice on grammar from a Rain Man? Well, sure, if you need it—after all, it knows all the rules, and does a pretty good job most of the time. But take it only as advice, and look over its shoulder while it counts the cards. Somebody has to be watching the dealer’s eyes. That job is better left to a human.
— James Lyle, Tester