What is life? Its vast diversity defies easy definition. (philosophy of biology)

[C C]

https://www.quantamagazine.org/what-is-l...-20210309/

EXCERPTS: “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.”

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. I once asked Radu Popa, a microbiologist who started collecting definitions of life in the early 2000s, what he thought of this state of affairs.

“This is intolerable for any science,” he replied. “You can take a science in which there are two or three definitions for one thing. But a science in which the most important object has no definition? That’s absolutely unacceptable. How are we going to discuss it if you believe that the definition of life has something to do with DNA, and I think it has something to do with dynamic systems? We cannot make artificial life because we cannot agree on what life is. We cannot find life on Mars because we cannot agree what life represents.”

With scientists adrift in an ocean of definitions, philosophers rowed out to offer lifelines.

Some tried to soothe the debate, assuring the scientists they could learn to live with the abundance. We have no need to zero in on the One True Definition of Life, they argued, because working definitions are good enough. NASA can come up with whatever definition helps them build the best machine for searching for life on other planets and moons. Physicians can use a different one to map the blurry boundary that sets life apart from death. “Their value does not depend on consensus, but rather on their impact on research,” the philosophers Leonardo Bich and Sara Green argued.

Other philosophers found this way of thinking — known as operationalism — an intellectual cop‐out. Defining life was hard, yes, but that was no excuse not to try. “Operationalism may sometimes be unavoidable in practice,” the philosopher Kelly Smith countered, “but it simply cannot substitute for a proper definition of life.”

Smith and other foes of operationalism complain that such definitions rely on what a group of people generally agree on. But the most important research on life is at its frontier, where it will be hardest to come to an easy agreement. “Any experiment conducted without a clear idea of what it is looking for ultimately settles nothing,” Smith declared.

Smith argued that the best thing to do is to keep searching for a definition of life that everyone can get behind, one that succeeds where others have failed. But Edward Trifonov, a Russian‐born geneticist, wondered if a successful definition already exists but is lying hidden amidst all the past attempts.

[...] Some philosophers have suggested that we need to think more carefully about how we give a word like life its meaning. Instead of building definitions first, we should start by thinking about the things we’re trying to define. We can let them speak for themselves.

These philosophers are following in the tradition of Ludwig Wittgenstein. In the 1940s, Wittgenstein argued that everyday conversations are rife with concepts that are very hard to define. How, for example, would you answer the question, “What are games?”

If you tried to answer with a list of necessary and sufficient requirements for a game, you’d fail. Some games have winners and losers, but others are open‐ended. Some games use tokens, others cards, others bowling balls. In some games, players get paid to play. In other games, they pay to play, even going into debt in some cases.

[...] A group of philosophers and scientists at Lund University in Sweden wondered if the question “What is life?” might better be answered the way Wittgenstein answered the question “What are games?” Rather than come up with a rigid list of required traits, they might be able to find family resemblances that could naturally join things together in a category we could call Life.

In 2019 they set out to find it by carrying out a survey of scientists and other scholars. They put together a list of things... (MORE - details)

[Syne]

Motivated reasoning is the only thing that makes the definition of life so broad or indistinct. Whether it's the fruitless search for extraterrestrial life or the desperate need to believe in abiogenesis or justify abortion (all contradicting each other), it's the need to justify their own preclusions that leads to any doubt of what life is. That's not science. That's wishful, and in some cases magical, thinking.

[Yazata]

Good thread CC. Thanks for starting it.

EXCERPTS: “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.”

Many of our concepts are like that. Try defining 'religion' or 'art' or 'consciousness'.

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.

Are black holes stars? Are neutron stars stars? Are brown dwarfs stars?

I once asked Radu Popa, a microbiologist who started collecting definitions of life in the early 2000s, what he thought of this state of affairs.

“This is intolerable for any science,” he replied. “You can take a science in which there are two or three definitions for one thing. But a science in which the most important object has no definition? That’s absolutely unacceptable.

Well, we have an established set of paradigmatic exemplars of what 'life' means: Earth life.

Complications arise when we start looking at the diversity within that class. Bacteria, archaea and the procaryotes. Single celled and multicelled eucaryotes. Autotrophs and heterotrophs. And viruses are obviously a glaring problem case.

But the real complications start to arise when we start thinking about hypothetical extraterrestrial life. It's possible to imagine life that isn't organized into cells, has entirely different chemistry and doesn't use dna or rna at all.

How are we going to discuss it if you believe that the definition of life has something to do with DNA, and I think it has something to do with dynamic systems? We cannot make artificial life because we cannot agree on what life is. We cannot find life on Mars because we cannot agree what life represents.”

My own view is that the best way to address that problem might be to think of life functionally. What kinds of things is it doing?

That's how we might best be able to recognize totally alien life as being life in the first place. Whatever its anatomy, physiology and biochemistry, to be life it will need to be doing things that we recognize as life functions. Utilizing its environment to make more of itself most obviously. Evolving by natural selection. Stuff like that. But it won't be easy.

I suspect that if we ever encounter life out there among the stars, we might have real difficulty recognizing it as life at all. If it's too different from our paradigmatic Earth life, if it's performing different functions in ways too different than how Earth life behaves, human explorers might only label it 'life' by courtesy or by analogy.

With scientists adrift in an ocean of definitions, philosophers rowed out to offer lifelines.

Some tried to soothe the debate, assuring the scientists they could learn to live with the abundance. We have no need to zero in on the One True Definition of Life, they argued, because working definitions are good enough. NASA can come up with whatever definition helps them build the best machine for searching for life on other planets and moons. Physicians can use a different one to map the blurry boundary that sets life apart from death. “Their value does not depend on consensus, but rather on their impact on research,” the philosophers Leonardo Bich and Sara Green argued.

Other philosophers found this way of thinking — known as operationalism — an intellectual cop‐out. Defining life was hard, yes, but that was no excuse not to try. “Operationalism may sometimes be unavoidable in practice,” the philosopher Kelly Smith countered, “but it simply cannot substitute for a proper definition of life.”

Operationalism of the sort described here sounds kind of circular. We define life as whatever triggers our best instruments for detecting life... That's not going to help us if we don't already know what to look for.

Smith and other foes of operationalism complain that such definitions rely on what a group of people generally agree on. But the most important research on life is at its frontier, where it will be hardest to come to an easy agreement. “Any experiment conducted without a clear idea of what it is looking for ultimately settles nothing,” Smith declared.

Yes, that's the problem as I see it.

Smith argued that the best thing to do is to keep searching for a definition of life that everyone can get behind, one that succeeds where others have failed.

That doesn't sound right either. I'm not really an essentialist who believes that things have essences that a correct definition strives to capture.

But Edward Trifonov, a Russian‐born geneticist, wondered if a successful definition already exists but is lying hidden amidst all the past attempts.

[...] Some philosophers have suggested that we need to think more carefully about how we give a word like life its meaning. Instead of building definitions first, we should start by thinking about the things we’re trying to define. We can let them speak for themselves.

Yes, I agree with that. But the difficulty, perhaps a fatal difficulty, is that when it comes to life, we only have a sample size of one (life here on Earth). That's not going to be very helpful when we are confronted with the exceedingly alien problem cases that I'm reasonably confident exist out there.

A group of philosophers and scientists at Lund University in Sweden wondered if the question “What is life?” might better be answered the way Wittgenstein answered the question “What are games?” Rather than come up with a rigid list of required traits, they might be able to find family resemblances that could naturally join things together in a category we could call Life.

Yes, that might be the best that we can do and I expect that it will be what happens.

[Syne]

Many of our concepts are like that. Try defining 'religion' or 'art' or 'consciousness'.

religion - various systems of faith
art - creative communication
consciousness - awareness

Are black holes stars? Are neutron stars stars? Are brown dwarfs stars?

Remnants of stars, so a stage in the life of a star.

Well, we have an established set of paradigmatic exemplars of what 'life' means: Earth life.

Complications arise when we start looking at the diversity within that class. Bacteria, archaea and the procaryotes. Single celled and multicelled eucaryotes. Autotrophs and heterotrophs. And viruses are obviously a glaring problem case.

But the real complications start to arise when we start thinking about hypothetical extraterrestrial life. It's possible to imagine life that isn't organized into cells, has entirely different chemistry and doesn't use dna or rna at all.

Imagined is effectively wishful thinking. And viruses are only a problem if motivated to make excuses for how they lack properties of all other known lifeforms.

My own view is that the best way to address that problem might be to think of life functionally. What kinds of things is it doing?

That's how we might best be able to recognize totally alien life as being life in the first place. Whatever its anatomy, physiology and biochemistry, to be life it will need to be doing things that we recognize as life functions. Utilizing its environment to make more of itself most obviously. Evolving by natural selection. Stuff like that. But it won't be easy.

I suspect that if we ever encounter life out there among the stars, we might have real difficulty recognizing it as life at all. If it's too different from our paradigmatic Earth life, if it's performing different functions in ways too different than how Earth life behaves, human explorers might only label it 'life' by courtesy or by analogy.

The scientific definition of life is according to what it is doing, which is why viruses are found lacking.

Yes, I agree with that. But the difficulty, perhaps a fatal difficulty, is that when it comes to life, we only have a sample size of one (life here on Earth). That's not going to be very helpful when we are confronted with the exceedingly alien problem cases that I'm reasonably confident exist out there.

Behaviorally, it should suffice. I mean, unless alien life behaves like terrestrial rocks, it really shouldn't be all that difficult to recognize. Unless alien life is incorporeal, but science already has trouble addressing that notion anyway, so we wouldn't be at any more of a disadvantage dealing with such aliens.

[C C]

I once asked Radu Popa, a microbiologist who started collecting definitions of life in the early 2000s, what he thought of this state of affairs.

“This is intolerable for any science,” he replied. “You can take a science in which there are two or three definitions for one thing. But a science in which the most important object has no definition? That’s absolutely unacceptable.

[...] Well, we have an established set of paradigmatic exemplars of what 'life' means: Earth life.

Complications arise when we start looking at the diversity within that class. Bacteria, archaea and the procaryotes. Single celled and multicelled eucaryotes. Autotrophs and heterotrophs. And viruses are obviously a glaring problem case.

But the real complications start to arise when we start thinking about hypothetical extraterrestrial life. It's possible to imagine life that isn't organized into cells, has entirely different chemistry and doesn't use dna or rna at all.

How are we going to discuss it if you believe that the definition of life has something to do with DNA, and I think it has something to do with dynamic systems? We cannot make artificial life because we cannot agree on what life is. We cannot find life on Mars because we cannot agree what life represents.”

My own view is that the best way to address that problem might be to think of life functionally. What kinds of things is it doing?

That's how we might best be able to recognize totally alien life as being life in the first place. Whatever its anatomy, physiology and biochemistry, to be life it will need to be doing things that we recognize as life functions. Utilizing its environment to make more of itself most obviously. Evolving by natural selection. Stuff like that. But it won't be easy.

I suspect that if we ever encounter life out there among the stars, we might have real difficulty recognizing it as life at all. If it's too different from our paradigmatic Earth life, if it's performing different functions in ways too different than how Earth life behaves, human explorers might only label it 'life' by courtesy or by analogy.

Generalized, articulated conceptions of an _X_ are arguably fine for armchair thinkers in allowing them to speedily apprehend a or theorize about a larger picture of "what's going on" (and what's possible and what's not).

But a generalized conception of _X_ can potentially become clumsy and inadequate if it isn't replaced by or narrowed down to the nomenclature slash precise meanings of specific expertise at the local level of workers/researchers. Tailor-made for whatever project or item of study that they're laboring on.

With respect to this subject, such might be applicable to sub-departments of biology rather than the enterprise considered as an umbrella category or whole. Future if not also current astrobiologists, for instance, doubtless would have to be more open-minded about life's borders than some dictated constraints issued by their terrestrial-focused colleagues.

When it comes to certain areas of technological-based life, biologists could seem to be excluded altogether. However...

Obviously traditional biology is still participating when it comes to something like xenobots. And even self-constructing, autonomous robots of tomorrow could still be affiliated with inter-disciplinary "hybrid" fields (half-biology) that mimic traditional life aspects (like muscle structure/function) with technological materials.{*}

At first glance, extremely advanced simulated life (i.e., residing in computers or supervening on computational operations) might be another technological activity threatening to exclude biologists. But clearly the latter would have to be consulted or directly involved heavily when accurately modelling familiar, existing or past existing life forms. Every artificial reality wouldn't consist of totally exotic "Other" creations developmentally falling out of rule-based processes that exploited a technological substrate/environment in ways not available for organisms in the original, natural domain.

- - - footnote - - -

{*} The future of life: Although progress in developing AI is slower than was expected 40 years ago, it is still ongoing, and it is likely that we will have conscious and intelligent machines some time around 2020. Machine intelligence and consciousness will inevitably be different to our own human intelligence and there will be a lot of debate as to whether conscious machines are ‘alive', and what their rights and responsibilities should be. Sadly, there is little evidence that human nature has changed much since Roman times, when one of the forms of public entertainment was watching people hack each other to death in gladiatorial combat. Today, we have Robot Wars, in which remote-controlled machines do battle. The machines are clearly just machines, so there is no debate yet about their treatment. But future robots might have powerful AI, and some will be designed to look and feel just like real people, with polymer muscles covered in silicone rubber. We cannot be sure whether these will ever be used in Robot Wars; however, it would certainly be a great crowd-pleaser if they were human-like—with synthetic blood—the more gore the better as far as audience ratings are concerned. Many will argue that there is no moral problem with using AI robots in this manner because they are ‘just machines'. But if we do allow such use of androids, even if they do not have full consciousness, we will have stooped once again to the lowest level of human morality.

[Magical Realist]

“This is intolerable for any science,” he replied. “You can take a science in which there are two or three definitions for one thing. But a science in which the most important object has no definition? That’s absolutely unacceptable.

What better science could there be than one which studies a phenomenon that can't be explained or defined?

[Yazata]

Here's some papers on the definition of 'life' that I found particularly helpful

The first is an interesting paper that proposes three rather different definitions of 'life'.

https://www.academia.edu/download/435508...15-Def.pdf

1. The first is what the author calls 'Woese Life' or more understandibly 'Tree Life'. (Woese is a guy who pioneered study of procaryotic lineages through observation of their highly-conserved small-subunit ribosomal rna.) So this definition of life would be 'life as we know it, Jim' or life in terms of its phylogeny. Life with the same common origin (LUCA). Anything with 16S or 18S SSU rRna would be this "Woese Life", the justification being that it has a place on Earth's evolutionary tree of life.

2. The second is what the author calls 'Darwin Life'. This is anything that undergoes natural selection so as to gradually increase it's 'fitness', however that is defined. It would seem to presuppose reproduction and some environmental selection process. It's different than Woese Life since it needn't be part of the same phylogenetic tree as Earth life and needn't share the same ancestry. Chemical self-replicators on some distant exoplanet can be expected to undergo natural selection too. Interestingly, Darwin Life would include the viruses (which evolve), while Woese Life doesn't (no SSU rRna). But self-manufacturing robots might qualify as life according to this one.

3. What the author calls 'Haldane Life'. This is a physical (thermodynamical) definition of life that isn't original with Haldane, in which 'life' is defined as a system capable of maintaining itself far from equilibrium, in the face of externally increasing entropy. Possession of a metabolism, we might say. Life satisfying this physicists' definition could (and in my opinion possibly does) exist elsewhere in the universe without any common origin with Earth life. (If it happened once, there's no reason to think that it couldn't happen again.) The self-manufacturing robots would probably satisfy this one too.

The author admits that none of these three definitions of what 'life' means capture all of our intuitions about the meaning of 'life', even the strictly scientific ones, but they allow for enough precision that different people talking about life will have some idea of what other people are talking about and not just be talking past each other.

Another interesting paper is the one by Edward Trifonov mentioned in the discussion that CC posted.

https://www.tandfonline.com/doi/pdf/10.1...1010524992

What Trifonov did was assemble some 123 definitions of 'life' from the scientific literature. Then he did a computer search for how frequently substantive words (excluding words like 'and') appear in those definitions. The idea being that the most frequent words point to the most prevalent ideas in how scientists think of 'life'.

He discovered that the words fell into ten broad groups. Most common were 'life' and various near synonyms. Which is reasonable, since all of the authors would want to say what it is that they are trying to define. (Life, living, alive, biological...) The next most common were organization words. (system, systems, organization, organisms, organized, network...) Third most common were 'stuff' words (matter, molecules...) and a chemical category (process, metabolism, reactions...) Then a complexity category (complex, complexity, information...) Then a reproduction category (reproduce, replication, self-replication...) Then an evolution category (evolve, change, mutate...) Then words related to environment and to energy. Finally ability words (able, capable, capacity...)

And lastly a brief history of the attempt to define the word 'life'. (I personally find a historical approach to ideas the most enlightening.)

https://www.academia.edu/download/593187...ji7fov.pdf

The authors say, "Attempts to address the definition of living systems have often led to nothing more than phenomenological characterizations of life, which are in turn often reduced to a mere list of observed (or inferred) properties. These inventories are not only unsatisfactory from an epistemological viewpoint but may also become easily outdated and may fail to provide criteria by which the issue of life (and its traces) can be defined."

And I'll add that phenomenological/descriptive definitions are probably going to be seriously inadaquate in the broader astrobiological context when our sample size of life and its observed properties is restricted to a sample size of one on one planet. It wouldn't come close to characterizing life's possibility space.

[Syne]

Darwin didn't know much about viruses:

When Charles Darwin’s On the Origin of Species went to press in 1859, viruses had yet to be discovered – it would be another 40 years after publication before the ‘concept of viruses’ was proposed, and a century later before breakthroughs in viral research would provide a clear understanding of their genetic make-up, how they replicate and how they cause disease.
https://www.cam.ac.uk/research/news/what...-evolution

And the problem with presupposing reproduction (necessary to evolution) is that it fails to classify mules as living, which is ridiculous on its face.

Since a fundamental attribute of living things is that they grow, viruses are not life. Even presuming that the life functions they hijack from infected cells are somehow attributable to the virus itself, new virions are created fully-formed. They do not grow or develop over their the course of their existence.