Encountering life beyond Earth must rank as one of the most momentous out-of-this-world discoveries imaginable. Quite apart from raising questions about the particulars of alien life-forms, such a discovery could pose - or answer - questions on the nature of life itself. This goes far beyond the familiar territory of 'what might aliens look like' to more challenging questions such as what is life in the first place, and what - if anything - separates the animate from the inanimate?
We may tend to think of life as being, at least potentially, universal. That is, in principle there is no particular reason why life should only occur on planet Earth, any more than we should believe that the sky is the limit of the laws of physics or chemistry. It is not just science fiction, but science itself, that speculates on the possibility or probability of extraterrestrial life, a probability that has surely increased with the discovery of extrasolar planets.
We can get a feel for the extent of biological possibility from considering the similarities and differences between life-forms on Earth. Extraterrestrial life, we may assume, may have some general characteristics of the kind shared by life on Earth - such as metabolism, reproduction, predation and so on - but not necessarily the particular characteristics of specific species, such as number of legs, or number of sexes. Overall, one could imagine alien life-forms that are not animals, not plants, nor fungi, bacteria or viruses, but are things at least as different from any of these as those are from each other.
The perception of what is recognised as a life-form must be partly dependent on what varieties of life exist in a particular world, and how different they are from non-living things. An alien society consisting only of animal-like creatures, on encountering Earth, might find it hard to believe that plants - strangely passive, brainless fixtures - were 'alive' in any meaningful sense, and not just part of the scenery, like rocks and waterfalls.
Similarly, we might ourselves find, on encountering some other world, a kind of biota that changed our own perception of life, by filling in some of the spectrum of possibility between life and non-life. Perhaps in some alien world, there is some kind of mineral kingdom, populated by crystalline life-forms, who are accorded living status just as surely as any virus or fungus. Their appearance as intermediate forms between what Earthlings recognise as organic or inorganic would help to bridge between previously distinct categories of animate and inanimate, just as the knowledge of monkeys helps bridge mentally between mice and men.
And imagining a most finely graduated spectrum between animate and inanimate, it might be difficult to recognise 'life' as a definitive, exclusive state after all.
On the one hand, this would accord with a philosophy that there is a degree of 'life' or 'spirit' in all things, whether rocks or clouds, forests or fruit-bats. This perspective accords, indeed, with some human belief systems, although it has not (yet) been assimilated by science.
On the other hand, if there is really no definite separation between the living and the non-living; if the transition between the animate and inanimate is so subtle or spurious that any dividing line is effectively arbitrary, or even illusory, then this removes any special status being accorded to the living - an anti-mystical position that surely accords with scientific tradition.
Perhaps the whole of nature is just one big mechanism after all. Perhaps biology is no less 'mechanical' than physics: the electric pulses in brains and muscles obeying orderly laws just like planets orbiting the sun, or electrons orbiting the nucleus. After all, it is the same physics and chemistry in living bodies as in inanimate matter. Our bodies are made up of elements like calcium and carbon, zinc and iron, which are the same whether in one context viewed as cosmic dust, or in another, lunch.
In other words, we are made of the same stuff as everything else: there is no separate 'vital' substance in our bodies; no special ingredient, not found in the periodic table, that separates the living from the non living matter.
So, just as science has convinced us that the Earth and the rest of the universe are made of the same stuff, and that humans and animals are of the same stock; perhaps we can regard all matter similarly, without the illusion of a privileged status for living matter.
And so, while 'life' may be universal, the perception of life as a special state may conceivably be a limited anthropocentric perspective - perhaps paradoxically a delusion of the more self-aware animate objects.
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