What is the meaning of "life"? Last week I talked about "the meaning of life" but a few shifted inverted commas make today's question quite different, if no less important.

It is particularly pressing at the moment because of the fierce row raging over the proposed embryo bill. People are arguing about whether embryo research is essential to prolong human life, or is an attack on its sanctity. But the question of what life actually is, and why it should be so important, has been largely ignored.

For large parts of history, life was thought of as a kind of force, an elan vital. The idea goes back at least as far as Genesis, where God blew the breath of life into Adam. It also crops up in Mary Shelley's Frankenstein, the author having been inspired by the discovery by the early Galvanists that animal muscles would contract if an electrical current was passed through them. Many speculated that electricity could therefore be the source of life, and could even reanimate corpses.

That idea of a life force strikes most of us today as hopelessly naive, though it persists in New Age spirituality. It has been replaced by a scientific, biological notion, in which something is defined as living if it grows and reproduces.

However, there is a third idea of life which is perhaps more helpful for ethics, and can be traced back to Aristotle. In general, Aristotle's biology is not of much interest to us today, except as a historical curiosity. Although he deserves some credit for virtually inventing the discipline, his theories have hardly stood the test of time. For example, he argued that men with long penises were less fertile than those with short ones, because their semen would have to travel further and so would be cooler by the time it was ejaculated. Nonetheless, Aristotle distinguished living things into three categories in a way that is still useful morally, if not scientifically.

He thought that all living things have at the very least the vegetative qualities of growth and reproduction. For plants, this is the only kind of life they have. Humans and animals also have a "sensible" or "appetitive" character, meaning they have the capacity to seek things such as food and shelter for themselves. However, humans alone have the third, and highest, feature of living things: we are rational creatures. We don't just grow like plants, or have desires and act autonomously like animals; we can also deliberate about what we do and make rational judgments.

Aristotle's distinctions raise the question of why the mere fact that something is living should be any reason to treat it with special respect. Whether you think life is a kind of force or merely a biological function, is life really sacred if it is merely vegetative or appetitive? We don't usually think so. The fact that something grows and moves is not enough for us to give it the same moral respect as human beings, which is why everyone is happy to eat plants, and most will also eat animals - although many think we ought to make sure our livestock doesn't suffer needlessly.

Special moral consideration for life therefore turns out to be restricted to certain types of it. We may have reasons to value plants and animals, but the moral significance of rational forms of life is of an altogether different order. If Aristotle is right, it is a red herring to think that all life has value in itself.

Apply this to the big life-and-death issues of our time and you can see why there is such wide disagreement. For Aristotle's modern-day equivalents, what we need to protect is not just any old bunch of living tissue. Thinking, feeling human beings always take priority over senseless clusters of cells, which is why these modern Aristotelians do not think of early embryos as anything special, and why, for them, the option of terminating your own life if your mental powers deteriorate too badly is not an abhorrent one.

However, there are still many who take a very un-Aristotelian view, and think that human life is always sacred, even if it is at a stage where it has only vegetative or appetitive functions.

You'll probably have guessed which view I take. There's nothing special about being alive in a purely biological sense. What makes some life especially worth protecting is that it sustains reason, emotion and feeling. That is the moral meaning of "life", and it's a big clue to "the meaning of life" as well.