what is life?

[Mr. Tweedy]

How are you defining life?

Must it be capable of thought? Have a backbone? Must it have DNA? Will RNA do? Does a virus come under your definition? Many define the ability to reproduce independantly as life, so is your definition based on reproductive capability? Is a sterile creature not alive?
A prion protein can causes disease by making non-disease proteins change shape - is this reproduction or not?

Off the top of my head, I'll offer a tentative definition: Life is a system that thwarts entropy by incorporating non-living matter and energy (stuff) into itself.  It self-maintains and so is able to continue existing indefinitely as long as it has stuff to work with.  ("Indefinitely" refers to life itself, not to individual organisms.)

Water crystallizes when it freezes.  By freezing,  is it defeating entropy by going from a less ordered form (liquid) to a more ordered form (crystals)?

I'm totally fascinated by this question now, one I hadn't really thought to ask myself: What is life?  How do we distinguish between living and non-living things?  I'd love to hear more thoughts, and I'm sure there are people here who have some good ones.

[Holden]

An entity is alive if it metabolizes.

[FNH]

How about, "if it breeds its alive".

[wherethewild]

An entity is alive if it metabolizes.

I´m pretty happy with including that in the definition. By this viruses are not considered life (which I believe most scientists agree with, although I may be very wrong here)

How about, "if it breeds its alive".

I believe breeding implies a mixing of genes to create offspring, which would exclude asexual reproduction.

[Holden]

An entity is alive if it metabolizes.

I´m pretty happy with including that in the definition. By this viruses are not considered life (which I believe most scientists agree with, although I may be very wrong here)

That's debateable. Though viruses do not metabolize on their own, they use the metabolic machinery of their hosts. They can't metabolize autonomously, but using their host, they can.

You're probably right about most scientists agreeing that virsuses are not life, but it's safe to say viruses do have some living characteristics. This may be a better question for a philosopher than a scientist.

[Thaurismunths]

I don't think you can sum life up as a single characteristic. How about:

Life has at least two of these characteristics:
Spontaneous Voluntary Motion
Metabolism
Reproduction

[Mr. Tweedy]

If I construct a robot that 1.) oils its own joints and 2.) builds other robots like itself, would my robot be alive?

Intelligence isn't an issue: My robot doesn't know or understand what it is doing.  It's just following its program.

[Holden]

I disagree with spontaneous voluntary motion being a characteristic of the living. Most plants don't move. I'm fine with reproduction being a trait of most living beings, but I would point out that not all living beings reproduce, mules for example.

[Thaurismunths]

If I construct a robot that 1.) oils its own joints and 2.) builds other robots like itself, would my robot be alive?
Intelligence isn't an issue: My robot doesn't know or understand what it is doing.  It's just following its program.

Self-sustaining isn't the same as metabolizing. A self-oiling machine doesn't metabolize the oil, it wears it out.
Also you bring up a good point that intelligence isn't a good justification because 1) we can't quantify it, and 2) many living organisms don't have any that we can qualify.

I disagree with spontaneous voluntary motion being a characteristic of the living. Most plants don't move. I'm fine with reproduction being a trait of most living beings, but I would point out that not all living beings reproduce, mules for example.

I used "Spontaneous Voluntary Motion" (SVM) to rule out rocks, cars, and robots. Robots can't move voluntarily, they can only react inside of what their program says they can do. Some plants can move spontaneously, and trees do exhibit SVM when younger by following the sun, but as their trunks develop rigidity they lose the ability to react drastically to stimuli as the sun, but they still turn their leaves over when it gets hot out.
I accounted for Mules and other non-reproductive creatures by making the criteria "Two out of three". Mules are capable of spontaneous SVM and Metabolism.

[Holden]

Conventional definition of life, courtesy of Wiki:

Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.

Organization: Being composed of one or more cells, which are the basic units of life.

Metabolism: Consumption of energy by converting nonliving material into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.

Growth: Maintenance of a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.

Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.

Response to stimuli: A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun or an animal chasing its prey.

Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.

[Russell Nash]

If we take out the organic components of the discussion, will we be able to eventually call AI lifeforms life? 

[Thaurismunths]

Really, reading Wiki's definition of "life" I don't think there's anything in there that says a living organism can't be man-made.
A complex mass of nanites could be considered "alive" if programmed to create more nanites as part of the same colony of nanites, can create offshoot colonies, metabolize organic and inorganic matter to power itself, respond to stimuli, adjust its environment to suit itself and adapt to itself to the environment.
AI wouldn't even have to be part of it.

Unless your question is "Can we have living AI that exists only inside a box?"
In that case, I dunno.
I want to say "No" because it would only be a complex program of simple questions.
In reality I think the answer is "Yes" because we aren't a whole lot more than a complex program of simple questions.

[slic]

Really, reading Wiki's definition of "life" I don't think there's anything in there that says a living organism can't be man-made.

Are you implying that life can't be man-made?  Should this be a factor? Not intentionally taking this into a religious place, but can only God/gods created life?

I've never given, "what is alive" any real thought.  I've never thought it important.  Whether or not something is alive has little impact on me.  It's more how it relates to me. For example, plants are alive, but I don't mind hurting them because I can't empathize with their pain.  Same with rocks.  Animals and other people though exhibit pain as I understand it, so I am more careful.  How do we know that rocks aren't alive?  Dirt doesn't actually go anywhere, it doesn't decay the way other organisms do.

How did people decide that plants are alive?  Because they grow?  Because they metabolism the surrounding resources into energy?

Is an AI life just because it thinks?

Doesn't life require more of an environmental interaction?  So then Thaurismunths is right when he says No about AI in a box.

Perhaps life is made up of two parts:
Interaction with the environment and rudimentary (or better) self awareness.

[Russell Nash]

Not intentionally taking this into a religious place, but can only God/gods created life?

I'm going to put on my Moderator hat for a second and say that I believe we have enough religious threads going at the moment.  If anyone would care to comment on the God part of this whole question, can we please limit it to "Yes, God needs to be involved." or "No, I don't think a God has anything to do with it."  If someone would like to explore the religious part of the equation deeper, I think one of those other threads would be a better place.

[Thaurismunths]

Really, reading Wiki's definition of "life" I don't think there's anything in there that says a living organism can't be man-made.

Are you implying that life can't be man-made?  Should this be a factor? Not intentionally taking this into a religious place, but can only God/gods created life?

No, I mean to say that we can make "Life." All we have to do is define what "life" is, and make something that does that. In reality it's alchemy and we'll be building golems, but the line between a construct and a child can come down to splitting hairs (think Pinocchio).
 

Perhaps life is made up of two parts:
Interaction with the environment and rudimentary (or better) self awareness.

So maybe we need criteria for what is "life" and what is "alive"?

[slic]

If anyone would care to comment on the God part of this whole question, can we please limit it to "Yes, God needs to be involved." or "No, I don't think a God has anything to do with it." 

That works for me, I just wanted to touch on it because the whole Frankenstein idea would be central to the AI discussion.

No, I mean to say that we can make "Life."

Thanks for the clarification. 

So maybe we need criteria for what is "life" and what is "alive"?

So life is around reproduction and alive is around self-awareness.
I still think interaction needs to play a role - Rocks are considered lifeless in part because they just still there and do nothing.  At least when you cut a tree, it "bleeds".

The Pinocchio example is a good one.  I would consider him alive even in puppet form - he's a wood-based lifeform   Even as a kid, I thought that his getting to be human was a jip.

[sayeth]

An article by Cleland and Chyba poses an analogy to the definition of water. Before the acceptance of atomic theory, philosophers could only define water by its characteristics. Once an understanding of chemistry was achieved, however, we could simply and accurately define water as H2O.   By analogy, they argue that we don't know enough yet about biology to make a good definition of life.  Once we have an empirical understanding of life, then the definition will be obvious.

[wherethewild]

By analogy, they argue that we don't know enough yet about biology to make a good definition of life. 

I think we were on a good way in the discussion already where people were using biological definitions to attempt to define this. Sorry slic, but your points are, in comparison, simplistic. Rocks are considered lifeless because the don´t metabolize, reproduce, adapt, grow, maintain homeostasis (thanks Holden for posting the Wiki entry). Of course this does mean that effectively they " sit there and do nothing", but in context to this thread it is an unuseful definition at best. The vast majority of people would agree that plants are alive because they do have the above characteristics. You can´t argue something isn´t alive simply because you can´t empathise with what that life is. That´s a subjective reference point which can lead to all kinds of unpleasant things.

Self awareness is a product of a complex brain. Organisms such as amoeba do not have any form of brain, yet they will react to, and interact with, their environment. Can fish pose the questions "Who amI and why am I here?"- if not, are they self aware or just aware? Yet both of those definately are alive. Self awareness is not a definition of life, it is the description of a characteristic some highly developed organisms have.

This thread started because I posed the question to Mr Tweedy what the definition of life was so that we had a common starting point for another discussion on evolution. In that I was trying to lead to a "what is the simplest form which could be considered life" so that we could continue that conversation.

[slic]

Rocks are considered lifeless because the don´t metabolize, reproduce, adapt, grow, maintain homeostasis

So given the above criteria, AI in the Skynet-from-Terminiator sense is absolutely lifeless, but in the Data-from-ST:NG sense could be life, mainly because Data has a body.

That most people think plants are alive is great, but what about modern cities?  Computer models show that they do everything but reproduce.  Certainly poets and authors take about them as though they were living entities.  Now I consider them more like the shell of a snail than a lifeform unto themselves.  My point being there is a difference between asking "what is life?" and "what is the simplest form which could be considered life?" 

As for using that as a starting block for a discussion on evolution, you might be better off agreeing on the definition of evolution.  Languages are said to evolve, technologies evolve.  The idea of evolution is clear, and I think it's clear that in world today there is evolution.  What I think your looking for is how/what causes these changes to occur.

If this is intended to be more of an ID vs "pure" evolution, it's not about the simplest form of life, but more about how simple, disparate mechanisms (for lack of a better word) can come together for an unintended and surprisingly useful purpose.

[Thaurismunths]

Sayeth: Dead on. You are absolutely correct that we don't know even close to enough to reach a conclusion about the nature of "Life." With physiology, psychology, and technology advancing so fast this argument is like nailing jello to a wall. And the comparison to the nature of water is brilliant. Nice find.
However, nailing jello to wall is still fun because you get to use jello and a hammer!

Wherethewild (things are?): I'm withholding judgment on Amoeba's self-awareness. Only because Science is starting to prove that 'lower functioning' creatures have self-awareness. Amoeba may be self-aware, but lack the means to express it to us in any meaningful way.

SLIC: I definitely support your concern about Skynet and Data, though they aren't so different. Although Data is an golem and Skynet is a computer network, they are by all accounts "dead" things. Self-awareness as we know it doesn't seem to have anything to do with life and they do not have any of the biological signs of life that we have discussed.
There also seems to be a continuance of life as a definition of life. You can turn Data off, put him 30 different cans for 1000 years, then put him back together and have Data again. The same is not true for any life forms I know of.

PS. Evolution is a fine criterion for life, but let's not get in to the nature of evolution if we don't have to. Thanks.

[edit] spelling.

[eytanz]

There also seems to be a continence of life as a definition of life. You can turn Data off, put him 30 different cans for 1000 years, then put him back together and have Data again. The same is not true for any life forms I know of.

But is the fact you can't do that to a human a property of humans, or an indication of our current lack of knowledge of how to do it? The whole pseudo-science of cryogenics is based on the premise that one day we'll be able to do exactly that.

I'm curious no-one mentioned "Bicentennial man" yet, either the movie or the story it is based on. If you start with something that lacks all the criteria for life, and then add them one at the time, how does that affect your definitions? Is being alive a grandient property, so that you can arrange a heirarchy of how alive things are, or is it a discrete state, and you're either alive or not? Most people in this thread seem to treat it as the latter, but my feeling is that it's more of the former.

[Mr. Tweedy]

There is no gradient of aliveness.  With all due respect to the classic movie, you can't be "mostly dead."  Some things appear to be dead but are really alive, but that is only because their life processes have slowed down to such an extent that they are difficult to perceive.  Even with the simplest bacteria, there is no problem of discerning whether they are still living or have died.

Cities are not alive because they do nothing for themselves: All of their processes are driven by humans for human benefit.  If the people were removed, the city would be nothing more than an usually angular mountain range.  A community cannot be defined without reference to its independently living members, and so a community is not an organism.

[Mr. Tweedy]

I think my self-oiling robot is alive.

My robot operates in a junk-yard and is able to fashion the junk it finds into parts for itself.  It knows to take the oil it finds dripping from discarded engines and apply it to its own joints, and it knows how to draw power from the cars' batteries to recharge its own.  Thus it takes purposeful actions to maintain itself and keep itself going.

My robot also has the amazing ability to build more robots like itself.  It spends the majority of its time collecting junk parts and carefully machining them into little gears and wires, and it has a special "clean" compartment in its lower regions where processor and flash memory chips are manufactured.  After it makes and assembles the "baby's" body, it plucks the chips out of itself and plugs them into the appropriate places.  The baby switches on and starts going about the same business as its parent.

I don't see any reason why this machine would not be considered a living thing.

[ClintMemo]

I think my self-oiling robot is alive.

My robot operates in a junk-yard and is able to fashion the junk it finds into parts for itself.  It knows to take the oil it finds dripping from discarded engines and apply it to its own joints, and it knows how to draw power from the cars' batteries to recharge its own.  Thus it takes purposeful actions to maintain itself and keep itself going.

My robot also has the amazing ability to build more robots like itself.  It spends the majority of its time collecting junk parts and carefully machining them into little gears and wires, and it has a special "clean" compartment in its lower regions where processor and flash memory chips are manufactured.  After it makes and assembles the "baby's" body, it plucks the chips out of itself and plugs them into the appropriate places.  The baby switches on and starts going about the same business as its parent.

I don't see any reason why this machine would not be considered a living thing.

It is only a robot because the parts are together and you recognize it as a robot.   You could make an automated factory and probably satisfy every criteria that your robot satisfies.
It also begs the question of how many of the list of criteria does it have to satisfy before it is alive.  If it's clean environment gets damaged, is it suddenly dead?  If I fix it does it become alive again?  None of it's other behaviors would have changed.  It could even have been damaged in such a way that it did not know that it was broken and then fixed again.  It could have lived, died and lived again without ever realizing it!

[eytanz]

There is no gradient of aliveness.  With all due respect to the classic movie, you can't be "mostly dead."  Some things appear to be dead but are really alive, but that is only because their life processes have slowed down to such an extent that they are difficult to perceive.  Even with the simplest bacteria, there is no problem of discerning whether they are still living or have died.

Being dead is only one possible way to not be alive. A rock is neither alive, nor dead. I'll happily agree that being dead is a discrete notion. That doesn't mean that being alive is.

Cities are not alive because they do nothing for themselves: All of their processes are driven by humans for human benefit.  If the people were removed, the city would be nothing more than an usually angular mountain range.  A community cannot be defined without reference to its independently living members, and so a community is not an organism.

This is the inverse part of the problem with your logic that Clintmemo notes above. Just like you accept the robot as alive because it is built in a way that you find easy to concieve in human terms, you reject the city for the opposite reason. you say it isn't alive, because it is big, and it is made of components that are themselves complex and alive. But a human body contains thousands of millions of bacteria. If they all were removed, the human would quickly die. Do we exist for our own benefit, or for the benefit of the many bacteria we house? Or do they perhaps exist for our benefit? Who gets to decide that? Us or the bacteria?

It is possible to define life in a perfectly coherent way that limits it to things that are of the size and complexity of humans and below. The question is - why do so?