I recently finished reading Carl Zimmer’s newest book, Life’s Edge, which is largely a tour of recent and historical science focusing on marginal life forms, artificial life, and the beginning of life. The book ends with a chapter about the definition of life, and I wanted to share some thoughts on it here because it so clearly frames the need for what I’m trying to do.
Early in the chapter, Zimmer reviews the proliferation of definitions of life, all the same kind of causally vacuous light-and-heat definitions I mentioned in my intro to semiosis.
Life is an expected, collectively self-organized property of catalytic polymers.
Life is a metabolic network within a boundary.
Life is a new quality brought upon an organic chemical system by a dialectic change resulting from an increase in the quantity of complexity of the system. This new quality is characterized by the ability of temporal self-maintenance and self-preservation.
Life is the process of existence of open non-equilibrium complete systems that are composed of carbon-based polymers and are able to self-reproduce and evolve on the basis of template synthesis of their polymer components.
Life is a far from equilibrium self-maintaining chemical system capable of processing, transforming and accumulating information acquired from the environment.
The existence of the dynamically ordered region of water realizing a boson condensation of evanescent photons inside and outside the cell can be regarded as the definition of life.
Life is a monophyletic clade that originated with a last universal common ancestor, and includes all its descendants.
And a frank one:
Life is what the scientific establishment (probably after some healthy disagreement) will accept as life.
“It is commonly said,” the scientists Frances Westall and André Brack wrote in 2018, “that there are as many definitions of life as there are people trying to define it.”
The thing that really made me sit up and pay attention was that Zimmer hits on exactly the same hypothetical about the definition of life that I included in my post.
As an observer of science and of scientists, I find this behavior strange. It is as if astronomers kept coming up with new ways to define stars.
Not that it’s such a special or unlikely analogy, but it told me for certain that Zimmer was very recently asking scientists and philosophers precisely the question I’m purporting to answer. I was very curious to see where he would land by the end of the chapter. He turns next to philosophy of biology, which was not entirely encouraging. I owe the existence of this Substack in large part to my ongoing annoyance at a single philbio paragraph about precisely this question.
Among the philosophers he surveys, the greatest focus is given to Carol Cleland, who argues that the entire question is futile because life is an empirical category. It cannot have a definition because definitions are explications of human-imposed terms.
If you’re a man and you’re unmarried, you are—by definition—a bachelor. Being a man is not enough to make you a bachelor, nor is being unmarried. As for what it means to be a man, well, that can get complicated. And marriage has its own complexity. But we can define “bachelor” without getting bogged down in those messy matters. The word simply links these concepts in a precise way. And because definitions have such a narrow job to do, we can’t revise them through scientific investigation. There is simply no way that we could ever discover that we were wrong about the definition of a bachelor as being an unmarried man.
The way biologists had been going about defining life was inherently circular. They had a set of objects they recognized as alive and listed a set of traits common to all of them. Then they used that list of traits to evaluate whether viruses or other marginal entities counted as life and evaluated the definition based on whether they liked how that line was drawn.
The most fascinating thing to me, though, was the analogy to a nearly identical episode in the history of science.
Before the modern age of chemistry, alchemists tried to define water in the same way many biologists define life: by putting together a list of its qualities. Water is a liquid, it’s clear, it’s a solvent that breaks up other substances, and so on. Far from clearing up the mystery of water, however, this definition only got the alchemists into more trouble when they discovered that not all water is alike. Some kinds of water dissolved different substances and not others. So the alchemists gave different names to those waters. But then they got into even more trouble with their definitions when they watched water freeze or boil. Ice and vapor do not share the properties of liquid water. The alchemists were forced to declare that they were entirely different substances.
It proved impossible to define water by listing its properties, because those properties were conditional epiphenomena of an underlying causal process (the structure and interactions of water at the molecular scale) that could only be discovered empirically. Rather than defining life, Cleland says, we need to theorize it, as chemists did for water.
As a card-carrying member of the “pet definition of life” club, I was prepared to reject Cleland’s perspective as premature pessimism. But I think she is exactly right about the problems with defining life. I only differ in arguing that biologists have already carried out more than enough of what Zimmer calls “tedious measurements” to properly theorize life. We have the same kind of intricate causal descriptions of life at many scales that were needed to define water.
Zimmer describes the discovery of an eventual theory of life in speculative terms, as if there some specific process could someday be described in a lab that would reveal the special secret of life.
Some scientists today believe that a theory of life can emerge only from exacting measurements of living things. They are inventing tools to precisely measure the timing by which genes turn on and off, the rate at which cells grow, the interconnected links by which living things sense the world and make decisions about what to do next. It may take decades before these precise measurements reveal patterns that let scientists recognize a full-blown theory.
A theory of life may end up looking a lot like the theory of superconductivity. It may explain life as a particular configuration of matter that gets a special quality from the physics of the universe.
And, well, you know what I have to say about this by now. There is no new discovery waiting for biologists to find out in the world that could make the answer to this question any more obvious than it already is. Tens of thousands of people have spent their careers filling journals with the details of the theory of life. They quite literally just haven’t realized what they’re looking at.
At this point, additional detail specific to life as it exists on Earth can only distract from the bigger picture. The fundamental theory of life is quite simple. I flatter myself to think that I can perhaps offer a more clear and accessible presentation than it’s been given by previous authors, but the idea has been out there for decades.
The “special quality” Zimmer is looking for is nothing more or less than the local persistence of structures that connect causes and effects that are not otherwise linked by the “physics of the universe.” In a word: semiosis. Whenever and wherever that process becomes self-sustaining and self-perpetuating, all of the properties of life become possible, though they may not all be present in every case.
Testing this theory by asking whether it correctly sorts edge cases in and out of life according to our preconceived notions makes as much sense as choosing a definition of water that excludes ice. Viruses, radio signals, computers, and spiderwebs are not composed of living cells, but they are all expression of the same causal process: the semiotic chain-reaction that distinguishes our planet from the rest of the known universe.