I bet every biology student has struggled with the introductory "what is life" chapter, full of conflicting definitions.
It is so difficult to define life because there is nothing to be defined. There are complex stuff and simple stuff. We tend to call things that exhibit a certain degree of complexity "alive", but that's just a word.
Can't remember who wrote the perfect definition of Biology: "The study of complex things". There's a great deal of wisdom in that.
Yes, I logged in to say the same. There can be mutliple definitions of life, each useful for different purposes.
It is like those people trying to figure out how many types of cells there are in the human body. There is no "right" or "true" answer, only useful ones. All these categories are artificial.
I bet a super-intelligent alien ethologist would interpret our society as a living thing.
I'm almost sure there are anthropologists who share the same view :)
The study of multi-organism systems: populations, ecosystems and the like is commonly associated with Biology, or a branch (Ecology). They will go as far as "biosphere", in an attempt to study the entire planet.
I was going to joke it's about medium sized things but size is not a reasonable distinction (neither absolute size nor number of "parts") and you quickly run into the problem how organisations (i.e. groups of humans acting together) or machines (e.g. a car) are fundamentally different.
I guess the definition really is pretty fuzzy and arbitrary. It's "obvious" why a bunny is biology and a car or company is not, but it's not quite so obvious why a virus or an android isn't.
Heck, xenobiology seems to be inherently problematic because alien life would defy all classification. Imagine the "xenomorph" from the Alien universe. Would that be considered a reptile? Is it even a vertebrate? What animal rights legislation would apply to it if any? Would a "grey" count as a person? We like to justify these laws with biological categories but what do we do when faced with situations where the categories break down?
EDIT: Is an artificial, viable sheep-human hybrid an animal or a human?
> We like to justify these laws with biological categories but what do we do when faced with situations where the categories break down?
Change them to suit the world as we understand it.
The reality is that classifications as strict rules are a fantasy we choose to believe in, but at the end of the day, classifications are descriptions, and so we have to mutate them to help us describe things as best we can.
The reality is we work to the level of detail and accuracy we need. Rigidity in labels and classification is probably a sign of a lack of understanding, because almost everything ends up being broken down more at some point, and exceptions are found that break the rules.
>Human society is complex, but it would not normally be considered biology.
Reality is organized in levels of complexity. Society is composed of living creatures, so at least you should say it is life-based. The big debate is how and where you draw the line between the non-living and the living.
That biology can be called 'the study of complex things' does not imply that all complex things fall under biology. It's silly to even have to point this out.
No, it's not a formal fallacy. "Apples are red, therefore everything red is an apple" would be. But the word "is" here being used to indicate definition, not characteristic.
What if we could use the notion of entropy in a definition of life?
Living things use energy to prevent entropy (decay?) while they are alive.
On the other hand, natural, non-living processes do no such thing, and increase entropy as governed by the law of thermodynamics.
We could also use criteria such as growth, reaction to stimuli, and reproduction. However, crystalline structures also grow and chemical processes may produce more of a certain molecule. All of those seek some energy minimizing equilibrium without any mitigating mechanisms.
You can't beat entropy, or prevent it. All you can do is change its distribution.
So, one way to look at living things is that each one is a local depression in entropy. But because you can't make something from nothing, each living thing has to broadly increase entropy in their environment, in order to decrease it locally.
Thus if we envision entropy as a surface, a living thing is not a depression from a flat plane, but rather more like a hill with a steep depression in the middle.
But again, since you can't have something from nothing, life can't arise from an environment of maximum entropy. In order for life to come into being, local entropy must be low enough for a hill to develop. This is where the entropic definition gets into trouble, because the local low-entropy environment around here is provided by the sun. And if you define life as any local depression of entropy, then it raises the question of whether the sun is alive.
Maybe it's better to think of life as a system, not an attribute. To use an analogy, you wouldn't look at a single wave in a rapid in isolation, and wonder what it is. Without the broader context of the river, you'll never understand how the wave came to be there. If every living thing is a like a ripple in the local entropic surface, we can point to individuals, but could only really understand them if we consider the whole picture of the solar system.
That's true - and it's been used in a few contexts.
The issue is the context. i.e. Living systems do take in energy and organize. However, if you make the box a little bigger, it's still subject to entropy.
I never understand why viruses are not life. Bacteria are life because they are essentially a single-cell organism and act like any cell in our body: they consume energy and produce waste. Viruses may not be a cell, but they can be damaged, they die, and they appear to be more than a simple chemical process.
Words are only useful insofar as they point to groups of entities that the speaker and listener agree are being pointed to. So, we should ask "what is the most useful definition of life?" It's a short word, so it's valuable real-estate. What should the definition of life be?
It's easy to get wrapped up into discussions about words, but don't forget that the things they point to is what's really interesting.
I agree. I think a source of confusion is the following.
Everyone has built up, over their lifetime, an implicit mental notion of "life", which is based upon the sorts of characteristics of things they classify as being alive. Many (I'd say most) people think this mental notion is some objective category that exists external to their mind, some thing that has a kind of essence that is shared by all the instances of life. They see the task of defining life as defining the essence of this category, and see there being an objective question as to whether something (e.g. a kind of virus) fits this category.
But this is to confuse a mental construct with what is out there. There are the objective properties of things like different types of viruses, different types of bacteria, of different types of mammals, and of all other natural phenomena. You can talk about what the similarities and differences are between the objective properties of a type of virus and a type of bacteria, for example, and there are simpler kinds of phenomena vs more complex ones, but there's no actual "life" category out there, separate from our mental constructs, to be objectively defined.
>Sb trying to push sb else to use a definition meets the second part of the sentence.
This is post-modernism in thin disguise.
Certain categorizations can be more valuable (practical, useful, just, etc) than others, in which case it's reasonable to rally for their use over other, less valueable, categorizations.
Some categories are just completely impractical, for example the category of "all objects except for my dog and its kibble, and excluding those that came in to existence on an even-numbered Gregorian year." Language is an application of signaling theory and there are lots of statements we can make about it. (For example it would be a bad idea to re-define the word "good" to mean that one above.)
It's post-modernism only if you assume that human beings have no set, general psychological characteristics and are entirely the product of socialization. Otherwise it is pragmatism.
It's most assuredly post-modernist because it implicitly assumes that all categories are equal in value (i.e.: that no category is better than another), and that the decision is at it's core a power-struggle (i.e.: oppressive to the loser).
Our understanding of life, viruses, and the relation between them is incomplete which means our definitions and lexicon as they are interrelated is also lacking in resolution.
Before the discovery of these Giant Viruses as I guess we are calling them now, you used to be able to make a useful distinction between cellular-based life, and viruses. Viruses were mere packets of information as far as our best understanding went with very simple mechanics. Cells are more complex, with a useful distinction being made between prokaryotes and eukaryotes when necessary, but each could be easily identified as being made up of one or more cells.
As our understanding of life improves, so does our useful observations into that murky grey area between life and not-life, and I suspect we will have to potentially extend the definition of life but come up with a vocabulary that more usefully delimits the forms it can take from its most simplest forms into the murkier grey area where viruses start to resemble cell-based life.
Science is a work in progress, and so must be the scientific lexicon. Consider further your use of the word "die" and is that really the word you would use when a virus expires? Maybe, if you consider them to be alive in the first place, but you might use the word "destroy" if you considered them to be more like an object, or at least not alive.
From David Goodsells The Machinery of Life:
"In 1944, the physicist Erwin Schrödinger presented a very simple definition of life that has withstood the test of time. He identified one property that all life shares: living things avoid decay into equilibrium."
Neither do rickettsia or chlamydia, two bacteria. Honestly, whoever was deciding on the definition of "what is life" knew ahead of time the kinds of things they wanted to include and they worked backwards from there.
We want to exclude viruses because they are too mechanical. There's no magic to them. That's the way it appeared before we started finding all these giant viruses anyways.
So viruses are data but living things are code. Viruses are just malicious data that exploits unintended behavior of the code to replicate itself. Of course all code is just data.
It's similar to how it's difficult to argue whether a data format is a programming language (Turing completeness is an obvious but unnecessary and insufficient distinction). "Declarative programming languages" muddy the waters but most people would agree that a JPEG image is just data whereas a PHP script is a program.
Humans also survive by ingesting other life forms (plants, animals). We couldn’t survive without vegetation either, because it converts our waste products into useful stuff (e.g. CO2 to oxygen).
Ingestion isn't really reproduction though, even if the dependency is there. I'm not saying it's a perfect definition, just pointing out that it's never been "consume energy, produce waste".
but it's unclear that this definition really means anything, because you can say that humans lack the facilities to replicate itself, without access to food and water and air too!
We have the organs to reproduce provided that we, you know, exist... I'm not arguing how correct the definition is, more that it's what is 'textbook' from my understanding, and not really do do with the consume energy, produce waste comment or ability to react to an environment like some other posts suggested.
A virus is just a computer program that needs a Von Neumann machine (CPU) to run on. Following this analogy we could define alive by saying that the combination of a program and a Von Neumann machine makes the entity alive.
Except that they really do have a physical manifestation (from the article, the cylindrical tail, the polyhedral box containing the genes, the fibrils on the outside). In some way they are closer to being the USB stick containing the computer program, that can reproduce onto other USB sticks but needs a host computer to do so.
Everything[0] has a physical manifestation. A computer program has a physical manifestation on its storage medium.
[0]: Let's not argue about whether it makes sense to say that abstract concepts also have a physical manifestation. Programs are concrete expressions encoded on a physical medium (whether using magnets, electrical charges or even the position of mechanical switches).
Bacteriophages attach to certain receptors, they don't respond to any stimuli.
The entire process of injecting their RNA is simply a chemical reaction to being attached to a receptor.
Basically, they just drift along until they hit a bacteria with the right receptor, then inject their genetic material and the empty hull is either discarded or simply continues to hang onto the cell.
>We don't consider something "alive" if its behavior doesn't change when its environment does.
I think this is too vague. A cat may continue to sleep without responding to changes in its environment but may respond to an internal stimulus (hunger) and get up and go. So response or lack of response to the environment may not be a good criterion to define life.
I think you're reading this too narrowly. The cat will respond to changes in its environment, even when sleeping. Try raising or lowering the temperature by an extreme amount while the cat is sleeping -- at some point it will awaken and seek a more comfortable place. To what extent does one need to change a virus's environment before it changes behavior? If its behavior is "fixed" by its genes and well understood, then there isn't much "dynamic" (complex) behavior being exhibited.
Perhaps when we consider that viruses are unable to form complex structures like tissues, organs and entire organisms, we must rethink if they're the same level of "life" as cells? I don't think a single definition of life is useful or might even exist, just as I don't think a single universal physical theory is possible.
The thinking on this has been changing recently as more discoveries are made. The linked article even alludes to it by calling viruses the fourth domain, implying that they are indeed life.
Trying to define "life" is one of the central questions of the philosophy of biology. And the positions one takes in the philosophy of biology are inevitably going to be influenced by one's metaphysical positions regarding the problem of universals. A nominalist, a Aristotelian realist, a Platonist (etc.), are all going to approach the question "what is life?" differently.
Sometimes when people discuss this topic, they assume a certain approach to metaphysics without making that assumption explicit (indeed, they may not be aware that there are genuine alternatives to their personal metaphysical positions.)
>"Trying to define "life" is one of the central questions of the philosophy of biology."
If this is true then "philosophy of biology" is a likely a total waste of time. It is obvious that life vs non-life is an artificial categorization that humans use because it is useful to them.
Your response presumes a particular philosophical position. When you call life "an artificial categorization that humans use because it is useful to them", you are espousing the nominalist solution to the problem of the universals, and applying that to biology. Even while attacking the philosophy of biology you are actually engaging in it.
Nominalism may be "obvious" to you, but it isn't obvious to everyone. Just because someone thinks something is "obvious" it does not automatically follow that it is true. Have you given serious thought to nominalism's alternatives, such as moderate realism (Aristotelian, Thomistic or Scotistic); or Platonism?
I consider legal entities such as corporations to be living organisms. So I don't agree that life is limited to organisms with a visible (biological) body.
In fact, corporations display many of the characteristics we typically associate with AI: perfect recall due to record keeping, ability to pay attention to many things at once, in theory immortal as long as it can keep up funding itself, can do projects that surpass the ability of any normal human to comprehend by itself, etc.
It's a bit slower than we regularly imagine computer-based AI to be, of course :)
>corporations display many of the characteristics we typically associate with AI...
I agree with your comment. This is how I think about corporations too. But I'd like to mention something about the name "Artificial Intelligence". I think that Artificial Intelligence is not artificial and it is not intelligent. I define AI as the ability of an organism to make right decisions to perpetuate its existence. Not only this is not artificial it is a widespread natural process. I also use it as the definition of life. An organism that can make the right definitions to perpetuate its existence is said to be alive.
Or maybe, more generally, the ability to make decisions.
Analogies are fun. The internet could be described as a brain. It performs many functions that affect real life, the information links and tags itself between different parts etc.
While true in principle, natural organisms tend to be vastly more complex than even the largest corps, and complexity leads to emergent behaviour that you wouldn't see in a hierarchical human organisation.
That's not really emergent behaviour. eg the decision making of an organisation is simply the decision making of it's personnel.
Complex organisations can self-organise even though the individual parts have little understanding of their position in the structure.
A better simile would be societies at large, and the changes in things like housing markets or stock markets, driven by individuals who are only guided by their own private interests, not those of the market as a whole.
>the decision making of an organisation is simply the decision making of it's personnel.
This is also true for animals, including humans. Most of our decisions are made by micro-organisms in our gut, by our microbiome. There are other decision-making centers as well. The self is not even aware in most cases that he/she is just applying a decision that was made for him internally.
>what do you mean by 'living', and why does a legal entity fit that notion of living?
Do I need to define "living"? I may just list some living organisms everyone agrees that they are living organisms such as animals, plants, and bacteria. Their common element to me is not that they have a visible body but their ability to make intelligent decisions to further their existence. So to me, to have a biological body is not a requirement to be alive. The only requirement is the ability to make intelligent decisions to perpetuate its existence as an organism. What I am saying is that life start as the result of an agreement between entities. Human body is the result of an agreement between the egg and the sperm. A legal entity starts as a legal entity. Why do you think corporations cannot be considered live organisms?
The Chinese room argument builds on the premise that there is a meaningful difference between "simulating" intelligence and possessing it.
If you "execute" the program manually, you may still not understand Chinese, but the program still does. The room, including you and the program, is functionally a single entity that speaks Chinese. The human is an implementation detail as much as the grey matter is an implementation detail in a Chinese speaking human.
Searle's argument that the machine does not actually possess "understanding" is simply special pleading. Unless he can define "understanding" in a meaningful way, the difference between an "understanding" entity and a "simulating" entity is non-existent.
These arguments all boil down to unscientific armchair philosophy. I have no idea why people seem to accept them as meaningful other than that the people who came up with them or wrote about them are otherwise highly regarded -- but that's literally an argument from authority.
Here's a controversial view, but any good comments section needs a few of them:
Viruses are no more "life" than bacteria are "life" or amoeba are "life" or ants are "life" or mice are "life" or humans are "life"... We're all just chemical machines, nothing more nothing less.
Giving the word 'life' some special significance is misleading and confusing. In order to not confuse ourselves we should be talking about functions and descriptions of those functions as implemented among chemical machines.
Do viruses reproduce? Yes, by hijacking cellular machinery. Do bacteria reproduce? Yes, by cloning themselves. ...
This sort of thinking about the world allows us to stay close to reality without abstract and ultimately meaningless distractions like 'life'. Heaving the burden of abstraction onto 'reproduce' is still dangerous, but much less so than allowing 'life' to lurk as an abstract category in our minds.
One of the (many and oft-debated) aspects of life is that via homeostasis it maintains a negative entropy gradient. That is, over time, it will coalesce more "order" within itself than the 2nd Law of Thermodynamics says should normally happen -- more than if it were merely a random bag of chemicals.
I like this definition, because it implies some kind of "trying" -- life must "try" to keep some balance that continues its existence.
(a) Viruses (and certain bacteria) confound this approach because they can indefinitely continue (and even evolve) their existence without actively maintaining or improving any kind of this "balance"; they rely on their host to provide the balance, or at least some critical parts of it (if by definition, they cannot exist outside a host).
(b) Does this make them life? Unclear, maybe.
(c) Could such an organism evolve to no longer depend on a host? Yes, conceivable.
(d) Would this organism that maintains homeostasis on its own be considered life? I would venture to say yes.
So at what point during the evolution from (a) to (d) did this organism become "life"? It seems like it is just semantics at that point. Pick your criterion, call it life.
I think a virus has much lower entropy than a cell. It's tightly packed, ordered and semi-crystal-like. Viruses make more viruses. I think they may be winning if you measure them by creating pockets of deceased entropy!
By that definition you could say that pretty much anything that does something is a machine. A computer is an electrical machine, the Sun is a physical machine ... We choose to give specific names like "computer" or "star" or "life" to physical phenomenons which fit a particular definition. It's mostly a way for us humans ot communicate and think about it. Life exists because we made up a word for it.
The problem with that analogy IMO is that machines can be comprehended by humans, whereas even the most primitive forms of life are more sophisticated than anything human created.
Reproduction is an astonishing feat in itself - if only we could create a machine which could reproduce, but it's not so simple!
Reproduction is easy, and was done in software, on the Internet, in 1988 [1]. There is a lot more to life than mere replication, for example homeostasis. The Morris Worm did not make any efforts to maintain homeostasis, and thus became extinct relatively quickly.
If we define reproduction as equivalent to how viruses reproduce (i.e. by repurposing machinery of cells to reproduce themselves), then we have created such machines; "built from scratch" human-made viruses exist consisting of synthetic DNA/RNA.
Is this not just picking a different abstraction? What is a “chemical” besides a collection of atoms? What is an “atom” except (some hand-wavey quantum stuff that I certainly don’t understand)? Why give primacy to the chemicals over the atoms or the quantum field?
We could go the other direction too, and consider it significant that we are a tiny part of a giant living body that is the planet. Does it stop there? Could we comprehend it if Earth itself was a part of some vast active living intelligent system, in the same way that we believe a bacterium or a cell cannot conceive that it is part of our own human body, seeing it simply as “the environment”?
It all gets fairly philosophical pretty quickly. I certainly don’t have any answers, but I will say that I find it a little suspicious how when we move far enough in any direction from the scale we ourselves operate on, our understanding of how things work devolves rapidly towards relatively simple deterministic or probabilistic equations, while our own life experience is rich and rooted in desires and decision-making.
While I totally agree with you, there is some value in having a clear definition of life if you're trying to do something like determine if it exists on other planets. In that sense, you're looking for some kind of self-separating chemical unit, which looks very different from a homogenous soup which happens to undergo complex chemical reactions.
From the philosophical side, one of the reasons we attach such abstractions to "life" is to avoid reducing it to a basic functional definition. i.e. we should not equate ant life with human life. Especially in the sciences, it's an important moral/ethical consideration.
1 micron is amazingly large. 300nm is about the threshold to resolve a particle under light microscopy, so these things would just be there - you could use a pipette and push them around.
anyone hear the theory that cytomegalovirus is a fragment of a gigantic prehistoric virus? a nurse friend heard about the theory in his medical microbiology class but I haven't been able to find much online.
My personal definition of life: self-replication with unlimited heredity.
self-replication: something can make a copy of itself.
unlimited heredity: when "information" is added, it is copied along, even if it does not contribute directly to the replication process itself.
Implied in the definition is variation. And in the bigger view, that all replicators are under pressure to do so well, or be outcompeted on resource usage etc., and disappear.
While a virus depends on a rich and active environment to work, they clearly fall into that definition.
Sortof. But with a few caveats. 1) most computer viruses don't randomly vary, and thus don't have a learning effect, miss unlimited heredity. 2) they are not physical life, so the relevance is not the same as their chemical counterparts.
It is so difficult to define life because there is nothing to be defined. There are complex stuff and simple stuff. We tend to call things that exhibit a certain degree of complexity "alive", but that's just a word.
Can't remember who wrote the perfect definition of Biology: "The study of complex things". There's a great deal of wisdom in that.