Remember the old line, ''If a tree falls in a forest, and no one is there to hear it, did it make a sound?"
Here's a variation: If a motorcycle goes down the street, and no one hears it, could it really be a motorcycle?
I mean, motorcycles are all about the sound they make, right?
So, what do you do with a motorcycle that makes no noise? Is it really a motorcycle?
I found myself wondering that, as I rode around recently on the ENV.
The ENV, or ''Envy," is an acronym for Emissions Neutral Vehicle . . . the world's first hydrogen-powered motorcycle.
It makes no sound other than a slight whisper of a breeze out of its cooling fins. And that lack of engine rumble is what's disconcerting about it. Motorcycles should make noise. Usually the more noise, the better. Unless you're a neighbor.
But even your neighbors would like the ENV. In addition to being ''whisper-quiet," which despite the public-relations hype about it, it actually is; it also is environmentally friendly.
The Envy will run for about four hours, or 100 miles, whichever comes first, on a ''tank" of hydrogen. The ''tank" is a detachable, modular fuel cell that is shaped sort of like an old 5-gallon gas can.
It plugs into the motorcycle's chassis, right where the engine would normally be.
The design seems clever because the same kind of modular tank could be made to plug into any number of other devices suitable for hydrogen power, such as a personal watercraft or ATV. It could even be a substitute for generator power, in something like a cabin or an RV.
The only question is ''where do you fill it up?" Right now, California has more solutions to that than most places. The state has two-dozen hydrogen refilling stations, with more on the way.
''In the none-too-distant future," said Harry Bradbury, the president of Intelligent Energy, ''people will be able to use a bike like ENV to leave work in an urban environment, drive to the countryside, detach the [fuel cell] and attach it to another vehicle, such as a motorboat, before going on to power a log cabin with the very same fuel cell, which could then be recharged from a mini hydrogen creator the size of a shoe box."
That scenario, with its level of convenience and interchangeability, is a bit farther off into the future than the introduction of the motorcycle.
The motorcycle is here now, and nearly ready to go. In fact, so eager to get started, Intelligent Energy is moving its headquarters to Los Angeles from London.
The bike was designed in Southern California at the Pacific Design Center in West Hollywood. It's a fairly straightforward albeit futuristic design, similar to a trail bike, with high ground clearance to give it both off- and on-road capability. It has decent suspension travel and a very favorable power-to-weight ratio.
Like a moped, it needs no shifting, because it uses a 6:1 ratio direct belt drive to deliver power to the wheels. More development could be done with gearing to achieve even higher top speeds, and/or greater economy.
In my spin around the parking lot on it, the bike was maneuverable and responsive. It would quickly and seemingly effortlessly run up to its top speed of 50 miles per hour. A zero-to-50 miles per hour sprint took about 12 seconds. It cornered nimbly and stably. I had no trouble slaloming between sets of cones.
Engineers asked me if I found it disconcerting that there is virtually no engine braking on deceleration.
But the brakes seemed to me more than equal to the task of stopping the bike, even though the brakes on the test model were lightweight bicycle types. In the production models, proper motorcycle brakes would be fitted.
Braking is also an area where even more economical operation may be found. In most hybrid automobiles, heat recaptured from the friction created by braking is a major source of energy regeneration. The Envy does not currently employ regenerative-style braking.
''But it could," Bradbury said.
The only thing that threw me was the near-total absence of sound. Again, I asked myself, ''Can it really be a motorcycle if it doesn't make any noise?"
Bradbury had, if not an answer to that question, perhaps a compromise: ''We call it a motorbike."