Defining Life

You may think it a simple question but an article by Ferris Jabr in the New York Times explains why defining exactly what "life" is is a devilishly difficult business. Every definition that scientists and philosophers have come up with either includes things we want to exclude or excludes things we want to include. The following excerpts from Jabr's piece begin with a very strange claim:

In fact, nothing is truly alive.

What is life? Science cannot tell us. Since the time of Aristotle, philosophers and scientists have struggled and failed to produce a precise, universally accepted definition of life. To compensate, modern textbooks point to characteristics that supposedly distinguish the living from the inanimate, the most important of which are organization, growth, reproduction and evolution. But there are numerous exceptions: both living things that lack some of the ostensibly distinctive features of life and inanimate things that have properties of the living.

Crystals, for example, are highly organized; they grow; and they faithfully replicate their structures, but we do not think of them as alive. Similarly, certain computer programs known as “digital organisms” can reproduce, mate and evolve, but ushering such software through the gates to the kingdom of life makes many people uncomfortable. Conversely, some organisms — such as gummy bear-shaped microanimals called tardigrades and brine shrimp (whose eggs are sealed up in little packets like baker’s yeast under the brand name Sea Monkeys) — can enter a period of extreme dormancy during which they stop eating, growing and changing in any way for years at a time, yet are still regarded as living organisms.

Why so much ambivalence? Why is it so difficult for scientists to cleanly separate the living and nonliving and make a final decision about ambiguously animate viruses? Because they have been trying to define something that never existed in the first place. Here is my conclusion: Life is a concept, not a reality.

To better understand this argument, it’s helpful to distinguish between mental models and pure concepts. Sometimes the brain creates a representation of a thing: light bounces off a pine tree and into our eyes; molecules waft from its needles and ping neurons in our nose; the brain instantly weaves together these sensations with our memories to create a mental model of that tree. Other times the brain develops a pure concept based on observations — a useful way of thinking about the world.

Our idealized notion of “a tree” is a pure concept. There is no such thing as “a tree” in the world outside the mind. Rather, there are billions of individual plants we have collectively named trees. You might think botanists have a precise unfailing definition of a tree — they don’t. Sometimes it’s really difficult to say whether a plant is a tree or shrub because “tree” and “shrub” are not properties intrinsic to plants — they are ideas we impinged on them.

Likewise, “life” is an idea. We find it useful to think of some things as alive and others as inanimate, but this division exists only in our heads.

Not only is defining life futile, but it is also unnecessary to understanding how living things work. All observable matter is, at its most fundamental level, an arrangement of atoms and their constituent particles. These associations range in complexity from something as simple as, say, a single molecule of water to something as astonishingly intricate as an ant colony. All the proposed features of life — metabolism, reproduction, evolution — are in fact processes that appear at many different regions of this great spectrum of matter. There is no precise threshold.

Some things we regard as inanimate are capable of some of the processes we want to make exclusive to life. And some things we say are alive get along just fine without some of those processes. Yet we have insisted that all matter naturally segregates into two categories — life and nonlife — and have searched in vain for the dividing line.

It’s not there. We must accept that the concept of life sometimes has its pragmatic value for our particular human purposes, but it does not reflect the reality of the universe outside the mind.

Whether all this is true or not surely Jabr's claim that nothing is truly alive is absurd. To use his own example it's like saying that because we have a hard time telling the difference between shrubs and trees therefore there's no such thing as a tree. One color shades into another on the spectrum, but it would be silly to insist that therefore there is no blue or red color.

Even if there's no clear demarcation between the living and the non-living it doesn't follow that no distinction is useful. Living things are structural entities comprised of proteins that are manufactured by nucleic acids capable of replicating themselves. Perhaps there are entities that fit this description that I would want to say are not living, but I'm unable to think of one. Nor can I think of anything usually thought to be living that doesn't fit this definition.

In any case, a video accompanying the article shows big machines created by a Dutch artist called Strandbeests which Jabr implies are alive. Although the machines are truly remarkable there's no reason to say they're alive other than that they appear to move in ways superficially similar to animals. Here's the video:

Thanks to Matt for the tip on the article.