As the debate over the ethics of human embryonic stem cell research has caught fire over the last year, there's been enormous discussion about whether scientists should be allowed to destroy human embryos in the process of getting these cells. Language like that portrays scientists both as murderers who take life and as Frankenstein-like usurpers of God's authority to make new life.

But when it comes to biology, words like "destruction," "creation," "embryo" and even "life" and "death" are ambiguous. Scientists understand this ambiguity to be a reflection of the complexity of living things. Meanwhile, both advocates and opponents of stem cell research are using that ambiguity to their best advantage. Take the term "embryonic stem cell," or ES cell. Scientists coined the term about 20 years ago -- ages before the current debate -- to describe a cell that could grow into all the cells that make up a human being. Of course, embryos can do the same thing, so what's the difference? To make sense of that, it helps to understand exactly what happens during both the earliest phase of embryonic growth and the laboratory development of ES cells -- something scientists are able to describe with some precision. When a human embryo is formed naturally through the fusion of an egg and a sperm, it goes through multiple rounds of division, and after a few days, the cells on the inside and outside begin to behave differently from each other. The cells on the inside are the ES cells. They -- and they alone -- will grow into the fetus and child. The "coat" of cells on the outside will develop into the placenta, the conduit for nourishment and waste removal that is discarded after birth.

In the natural course of development, ES cells don't last very long. They morph within two weeks into cells with a more restricted ability to produce a limited number of tissues or organs. In the laboratory, however, scientists have figured out how to remove an embryo's "coat" and get the ES cells to multiply indefinitely without morphing into anything else. Because of their versatility, ES cells are the darlings of biomedical research. All scientists need to do is to identify the molecular signals and cellular environment required to generate any tissue or organ of choice. But if lab-grown ES cells can be coaxed to develop into any one tissue, might there be an environment in which they could be coaxed into making a whole human body? The answer is yes.

Eight years ago, a Canadian embryologist named Janet Rossant began turning mouse ES cells into live-born mice. She started with a small bunch of ES cells, provided them with a fresh pre-placental "coat" and then placed them into a mouse's womb, where they underwent normal development and eventually became a normal newborn mouse. Many more mice have been developed from ES cells since then, and there is no doubt that the same protocol could work with human ES cells (which is not to say that there is any reason or need to do such a thing).

There's a word biologists use to describe a cell, or group of cells, that by itself can develop into a whole animal or person: That word is "embryo." Each random bunch of eight to 10 human ES cells is nothing more or less than a "naked" human embryo -- that is, an embryo without its pre-placental "coat."

Stem cell scientists have been less than eager to discuss this tidbit of embryological lore and its implications in public, because of the fear that equating ES cells with embryos could give ammunition to opponents of stem cell research. Their reluctance is likely to be even greater now that President Bush has drawn his own moral line between research on existing ES cells, which he deems acceptable, and direct research on embryos, which is deemed unacceptable. But it seems to me that the president's moral line has been drawn between a basket of unpeeled apples and a basket of peeled apples -- the difference between the two is in appearance only. With the political debate at fever pitch, I have no doubt that some scientists will challenge my interpretation here, but it will come down to an argument over words, not biology.

Ironically, the same interpretation of ES cell/embryo equivalence can be used just as readily by advocates of stem cell research to bolster the case for allowing federal funds to be used for extracting ES cells from unwanted embryos sitting frozen in IVF clinic storage tanks. ES cell research won't actually destroy these embryos, it will save them from an otherwise certain death. Not only can these embryos be preserved indefinitely (as living ES cells), but their use could lead to therapies down the road that will save the lives of many other people as well. Thus, lives will be saved without any lives being lost. As I write this, I'm fully aware that I, too, am using language in a way that best supports my position.

This brings me back to the question at the heart of the stem cell debate. Is a one-week-old human embryo -- a ball of cells smaller than a pinhead -- a form of human life that deserves our respect and protection? The problem with this question is that, of all the terms used in biology, "life" is the most ambiguous. And without a clear definition of "life," the question of respect is meaningless.

At this point you may be thinking, how hard can it be to distinguish between what's alive and what's dead? To which I say, consider what happens right after a man is shot to death with a bullet to the head. We can all agree that he is dead, but for at least a few hours, 99 percent of the cells below his neck are still very much alive. Indeed, his organs can continue to function for many years if they are transplanted into the bodies of other people, and some of his cells can survive forever in laboratory incubators. When I say that the person is not alive, it is in the sense that he no longer exists as a sentient being. But when I say his body is still alive, I am using the same word "alive" in a general cellular sense. Aristotle recognized the difference between vegetative life and conscious life more than 2,000 years ago, and physicians today commonly use the term "persistent vegetative state" to describe human bodies that have lost higher brain function and, with it, the potential to achieve a sentient state.

Is a one-week-old human embryo alive? The answer is clearly "yes" if we use the cellular or vegetative definition. And it is just as clearly "no" if we use the definition of sentience. Normally, we don't give much respect to cellular human life -- we shed skin cells without a thought -- so why should we respect a microscopic human embryo?

According to the Catholic Church, every human embryo deserves as much respect as you or I because it contains a God-given human soul or spirit. Indeed, when people ask whether an embryo is alive, often what they really want to know is whether it has a soul. But how do you calculate the number of souls in a petri dish containing 10 million human ES cells? The problem is that cells can be grouped together in bunches of eight, nine or 10 to form embryos, and depending on the size of the average bunch you make, the number of embryos in the dish at any moment can vary by hundreds of thousands. Counting embryonic souls is akin to counting the number of angels dancing on the head of a pin. Whatever the number, the idea that an embryo has a soul is a matter of religious faith, not science.

Until recently, the most persuasive secular argument for protecting embryos had been that embryonic cells are different in some fundamental way from all other cells in your body because they alone have the potential to form a sentient being. The assumption was that all other cells were irrevocably chained to the narrow task assigned to the particular tissue or organ in which they were placed.

But within the past three years, this view of cell biology has been proven false. Scientists have discovered the molecular keys required to unlock an amazing plasticity in cell identity. Brain cells have been turned into blood cells, fat cells have been turned into bone, muscle and cartilage, and other examples of cell conversions are flooding the scientific literature. Of course, none of this is referred to as cloning, although that's exactly what it is. It is only a matter of time before scientists uncover the mother of all molecular-conversion keys: the one that transforms an adult cell directly into an ES cell. In philosophical quarters, that discovery should be a lethal blow to the idea that potential alone is a sufficient criterion on which to base the granting of respect and protection: even skin cells will have the potential to become babies. In political quarters, scientists will claim that by bypassing embryos in their production of ES cells, they will have eliminated all ethical objections to the research and its applications. And in the world of medicine, clinicians will eventually be able to provide replacement tissues and organs produced from a patient's own cells, which will not be rejected as foreign by the patient's body.

Most scientists will continue to avoid the use of contentious terms like "embryos" and "cloning," and maybe opponents will as well. Instead, they will focus on more palatable terms like "cell therapy" and "tissue renewal." And it won't be the first time. The classic example of shifting terminology as a method of public appeasement was the substitution of "magnetic resonance imaging," or MRI, for the original scientific term -- "nuclear magnetic resonance," which was thought to be less than patient-friendly.

If taking the "nuclear" out of MRIs or saying "ES cell" instead of "embryo" results in a greater acceptance of the science, that will lead to a healthier population in years to come. And that's not such a bad thing.