Last week I was editing an article about the Russian space science program. I was reading along easily until one sentence stopped me short: "More than twenty different living biological species were studied during the BION space flights - rats, fishes, tortillas, insects, plant tissues, seeds etc."

Tortillas, as a living biological species? Only, presumably, very old and moldy tortillas. A quick exchange of e-mails to Moscow revealed that the word the authors were after was "tortoises." But it started me thinking. How do we know when something is a living organism?

At this point I would like you to stop reading, think for a while, and offer your own answer to my question.

Assuming that you have now done so, I will do my best to confuse you. Let's look at some of the ways that people define living things.

Did you say, "A living thing must be able to reproduce itself."? Women of a certain age might take exception to that. The definition excludes them from the realm of the living, as well as all children.

"A living thing is something that at some time in its life, even if not now, is able to reproduce itself." Sorry, that's not much better. Worker bees are all sterile females, but one of them is buzzing around my office as I write and it seems to be very much alive. A man lost on a desert island from childhood can't ever reproduce, either, no matter how much he would like to.

Moreover, viruses seem to do nothing but reproduce themselves, although one of today's top biologists, Lynn Margulis, says of viruses, "They are not alive since outside living cells they do nothing, ever." I don't go along with her statement. Everything that reproduces can do so only in the right environment, which for a virus happens to be the inside of a living cell. I regard viruses as alive. On the other hand, I don't believe that computer viruses, for which the right reproducing environment is your computer and the Internet, qualify as living objects.

"A living thing must be descended by reproduction from some other living thing." This is not only a weak definition - defining an object in terms of itself - it also has real problems because at some time in the past it can't have been true. In a universe of finite age there must have been a first living thing, even though we know nothing about when, where, or how it formed.

Let's put reproducing to one side and try another tack. "A living thing must be able to take external material from its environment and use it to grow." That certainly applies to everything from tigers to turnips. Unfortunately, it is too broad a definition. Take a crystal of copper sulfate, hang it in a strong copper sulfate solution, and go away for a few days. When you come back, the crystal will be a bigger crystal. It has taken what it needed from its environment (in this case the copper sulfate solution) and used it to grow.

Crystals, however, grow only on the outside. Also, they do not transform the material they use to some radically different form, as a plant transforms water and carbon dioxide to carbohydrates, or as we transform a pizza to another inch on our waistline. We give a name to this process of transformation, calling it metabolism. The source of energy can be anything from sunlight (for plants) to the breakdown of food (for animals). Living things must metabolize, and nothing else can. That's the best definition so far. But even here we have to be very careful. Certain spores and seeds can remain dried up and totally inert for many years, with no evidence of metabolic processes; but give them water and sunlight and they spring into action.

You might ask, why make such a big deal of out the living - non- living question? In practice we can tell always tell the difference. If not by the obvious rules - it's green, it grows, it moves, it eats, it's coming to get me - then at the microscopic level. We can examine the object closely to see if it contains the DNA or RNA molecules necessary for reproduction. If it doesn't have them, it's not alive and never was.

That's a strong, solid answer - for anything on Earth. But what about other places in the universe? Does a living thing, anywhere, have to use DNA and RNA to store its genetic information?

We don't know. There are molecular alternatives. In the next decade we will send more spacecraft to Mars, and later this century we will probably take a close look at Europa, where a great ocean seems to lurk beneath the ice-bound surface. These spacecraft will look for life; but what will they look for?

The Viking spacecraft taught us caution. In 1976, it landed on Mars carrying a chemical test kit designed to seek evidence of life. And life was what it apparently found - everywhere. So much so that scientists moved quickly from jubilation to skepticism. The tests were not specific enough. They confused other chemical processes with the signature of life. Next time around we will surely be more careful. But we may still be tricked by our own tunnel-visioned assumptions about what life must be like.

Answering the question, how do we identify a living organism, turns out to be harder than you might think. Life is a tricky and a subtle thing. Value the one you have, and hang on to it as long as you can.

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