Bill Poser discusses today in Language Log a deep ontological question: Is a cow a motor vehicle? A court in Ohio ruled that it isn't, because it lacks wheels (this is based on the American Heritage Dictionary definition of motor vehicle, which requires wheels). A cow is indeed not a motor vehicle, says Bill, but not because it doesn't have wheel (aren't tanks motor vehicles, he wonders with reason), but because it doesn't have a motor.

So here is a prime example of what an academic can do that a person of law can't: namely, answer a question without really answering it. Defining a motor vehicle as a vehicle with a motor then requires us to ask what a motor is; and we are essenitally back at the beginning. If we decide to refer to a dictionary now, by the way, we discover that a motor is:

1. Something that produces or imparts motion.
2. A device that converts any form of energy into mechanical energy.

According to this definition, a cow technically has a motor, and therefore (assuming that it's a vehicle, a proof left to the readers as homework) it is a motor vehicle.

Now if you are a judge, you are (at least supposed) to avoid this kind of tautologies. You know that a cow isn't a motor vehicle by any reasonable standard; deciding otherwise would be an abuse of the civil law. You can either justify it by using some version of the "reasonable person" argument; or you can try to reduce the problem to a less ontologically complex one, such as deciding what is a wheel, which seems a perfectly nice escape.