John Wilkins at Evolving Thoughts has a great post about the development of the modern definition of “Life” (which, one strongly suspects, is by no means fully developed). Once we break free of the most parochial definitions involving carbon-based chemistry, we’re left with the general ideas that life is something complex, something that processes information, something that can evolve, something that takes advantage of local entropy gradients to make records and build structures. (Probably quantum computation does not play a crucial role, but who knows?) One of the first people to think in these physical terms was none other than Erwin Schrödinger, who was mostly famous for other things, but did write an influential little book called What Is Life? that explored the connections between life and thermodynamics.
Searching for a definition of “Life” is a great reminder of the crucial lesson that we do not find definitions lying out there in the world; we find stuff out there in the world, and it’s our job to choose definitions that help us make sense of it, carving up the world into useful categories. When it comes to life, it’s not so easy to find a definition that includes everything that we would like to think of as living, but excludes the things we don’t.
For example: is the Milky Way galaxy alive? Probably not, so find a good definition that unambiguously excludes it. Keep in mind that the Milky Way, like any good galaxy, metabolizes raw materials (turning hydrogen and helium into heavier elements) and creates complexity out of simplicity, and does so by taking advantage of a dramatic departure from thermal equilibrium (of which CV readers are well aware) to build organization via an entropy gradient.
Update: Unbeknownst to me, Carl Zimmer had just written about this exact topic in Seed. Hat tip to 3QD.
Quasar: “As Sean has pointed out Mules are alive but cannot reproduce”
Aren’t mules made of DNA?
Sean’s golem may be fantastical but it demonstrates that the quality of awareness is sufficient to define life.
And now that it appears that self-organizing, helical, information-processing structure may be a feature of inter-stellar space, the golem or an SAS or Boltzmann Brain is not so fantastical after all.
re #27 Awareness doesn’t help much either. Is a bacteria ‘aware’?
I think the B.B argument (whether applied to organic or computer based “life”) is a negative demonstration of big bang nucleosynthesis and the theory of evolution.
It shows that the extraordinarily improbable is always overtaken by the near inevitable – biological life forms based on carbon will evolve into life under the right conditions
Long before a BB could emerge out of a chance quantum fluctuation, the universe would have moved to a state where molecular structure was impossible, unless there was a new big bang.
An infinite growth in organisation and information just can’t happen.
Also, we can think of whether a group of organisms that lives together form a “superorganism”. E.g. we humans consist of cells and are clearly organism in our own right. Is an ants nest also a superorganism? Or groups of people living together?
What about a sufficient condition for a system to be considered “living”. Barrow and Tipler proposed a system at least composed by subsystems capable of self-reproduction in some enviroment and containing information which is preserved by natural selection.
The Anthropic Cosmological Principle. §8.2
Dear Sean,
This is a great post, can I offer a definition of life from my first book, Life of the Cosmos, p 156:
“A living system is
A a self-organized, non-equilibrium system (see below), such that
B its processes are governed by a program which is stored symbolically, and
C it can reproduce itself, including the program.”
On p 155 I defined a self-organized, non-equilibrium system to be “a distinguishable collection of matter, with recognized boundaries, which has a flow of energy, and possibly matter, passing through it, while maintaining, for time scales long compared to the dynamical time scales of its internal processes, a stable configuration far from thermodynamic equilibrium. This configuration is maintained by the action of cycles involving the transport of matter and energy within the system and between the system and its exterior. further the system is stabilized against small perturbations by the existence of feedback loops which regulate the rates of flow of the cycles.”
The point is that the biosphere as a whole, and perhaps a stable disk galaxy may satisfy the definition of a self-organized non-equilibrium system, and, while that is interesting and worth noting, living organisms made of cells, satisfy the further conditions B and C. On the other hand, computers, computer programs, and robots do not satisfy A, even if they can be built to reproduce themselves.
One could also emphasize the existence of a semipermiable membrane, the flow of matter through which is under the control of the cell-as it is possible that the self-construction of such membranes played a key role in the origin of life, as discussed in adap-org/9709004.
Thanks,
Lee
re #28: I don’t know whether bacteria are “aware” although they could be and a discussion of what constitutes “awareness” would be an interesting next question. I personally find the term preferable to “consciousness” which carries a lot of baggage.
But my point was that awareness is a sufficient, not necessary, defining characteristic.
Lee, it is not so clear to me why robots (once they are fully capable of maintaining themselves and taking care of all the facilities that support them) would not satisfy A. It seems to me that if A is not satisfied then they would not be able to last very long…
it is not so clear to me why robots (once they are fully capable of maintaining themselves and taking care of all the facilities that support them) would not satisfy A
Not only material robots, but also software simulations of arbitrary complexity, with subsystems that are not at “equilibrium”. Does the containing system have to be our universe? Or is there something inadequate about Turing equivalence (perhaps properties that the universe has which Turing machines do not?), making “life” (as per Lee’s definition) impossible in a software environment?
Count, by robots I mean robots as they exist now, such as those that do repetitive tasks in factories. I do not claim it is impossible that we could construct artifical living things.
I read Lee’s book a few years ago and quite enjoyed its arguments, while not agreeing with them all. But I’d rather not thumb through it again. So I’ll just talk off the top of my head instead!
I don’t really see how galaxies can be described in any sense as “living”, any more than the sun is. They are just self organised collections of matter that are able to reduce entropy over very long timescales.
Only by a very circuitous argument could they be said to reproduce themselves, although there is a form of evolutionary cycle at work.
So let’s say that they form a precondition for life e.g carbon, oxygen & nitrogen nucleosynthesis and the supply of radiant energy against the background of cold space. Life is a thin film of exotic chemistry based on that.
I’d see any “awareness” that life has developed as an evolutionary mechanism, but not a definition. The sense perception mechanism would be better.
It enhances survivability, but is something an inanimate self organised system can’t have.
Once you can store sense perceptions and have the ability to process them, you have the rudiments of awareness – i.e. a mechanism where the organism is able to evaluate the effects of different courses of action.
As to the “robotic argument”, I agree it’s a theoretical possibility that we could construct artificial living things, but I don’t think the reverse is true, for the reasons outlined up the thread.
Hence it’s not an argument that the evolution of life, or even a spontaneous machine organism could result from such a mechanism. I suppose there’s a rather unsettling argument that in the interests of survival, humans might increasingly try to incorporate themselves into longer-living machine intelligences. That way lies the Borg……
I still don’t see why the ability to reproduce has anything to do with whether we would classify something as “alive.” I’m pretty sure I can imagine coming across a novel organism and judging that it was unambiguously alive without having any idea whether it could reproduce. Why is that property so important?
Otherwise, Lee’s definition does a pretty good job at formalizing the information-processing part of the definition. The “running according to a program” aspect is an interesting idea that I don’t think I’ve heard before. I would have to think about whether there could be counterexamples where there is no such program even though we would all agree that something was alive.
I’ve written about a dusty plasma physicists’ neat idea for life on Clifford’s blog here , where we discussed scalings for life: here and there was a related thread on CV here.
Life definitely includes things that can ask this question (“the meaning of life”), right?
The argurments against reproduction make sense for individuals, but how do you make sense of evolution (another one of your criteria) without some kind of reproduction? Your Golem doesn’t evolve — neither do I. Evolution usually refers to the change of an entire population/species, not an individual. And it always assumes the reproduction of new individuals within this group.
It’s very difficult for me to imagine how to produce something with the other qualities of life “naturally” if that something lacks the ability to reproduce (which it must have inherited), because the alternative appears to necessitate some kind of miraculous saltation from the primordial schmutz. From the get-go, if we all agree that “life” has a kind of…er…negentropic complexity that is self-sustaining, how could it possibly come to be without a miracle (or an inventor)? Evolution is all my poor little brain can come up with that fits what we know about the natural world. And if something doesn’t reproduce, how can it evolve? So, again, if reproduction isn’t part of the formula, I have a hard time imagining how there could be life at all. Sure you get a celibate or sterile individual here or there, but even if they don’t reproduce, they owe their very existence to reproduction if they are “natural”.
Always fun to ponder this question! I agree that before humanity can get serious about exobiology, we needs us an answer.
You may be interested in my paper “Emergence Explained” (Complexity, Sept/Oct 2006) and my 2 page abstract submitted to the upcoming Workshop on Philosophy and Engineering “Bits don’t have error bars.”
I can certainly imagine an individual evolving. Besides which, I’m by no means certain that evolution (in some Darwinian sense) is a necessary part of the best definition of life (which I don’t claim to have).
Duck-Rabbit illusion
NSF encouages Paradigm Shift
#37 Actually, I think the reproduction definition is essential to defining life, as we know it (Jim), whereas running a program is a highly questionable definition.
Programs are artefacts of created by pre-existent living creatures that have evolved and reproduce. Running a fetch-execute cycle of stored instructions doesn’t seem to be the way living things operate and nor is it proven that the universe (as a whole) is algorithmic.
Max Tegmark’s article/paper called the “Mathematical Universe” has some useful discussion on this topic and of course, it relates to Alan Turing’s original preoccupations with thinking machines.
Unless he operates under a pseudonym here, it’s a shame that Tegmark doesn’t participate in these discussions – I’d like to see a Tegmark, Carroll, Smolin smack down.
I think it’s wrong to include things like reproduction and evolution in the definition of life. These are essential mechanisms making it possible for organisms to have appeared naturally (without interference of intelligent beings), but not essential for a thing to be alive.
Why not just keep Smolin’s point B (see post # 37):
“its processes are governed by a program which is stored symbolically”
The “which is stored symbolically” is not necessary i.m.o.
If an organism is identified with a certain program then from the perspective of that organism, or from our perspective, it doesn’t matter much if you modify it so that it doesn’t satisfy the other criteria anymore.
I would still be the same person if I were replaced by a robot with a computer in its head that implements my neural network.
I made a typo, Smolin’s post is #31 not #37
Here is another stab at a definition from Jerome Rothstein
“In the broadest sense, he describes generalized intelligent life forms as self-replicating, computer-controlled heat engines that are able to play survival games.”
Note this definition is of “intelligent” life not just life.
Elliot
I’m going to go back to the reproduction argument, since I think this is central.
1) In defining an living organism, we need to think a bit more broadly than individuals.
The behaviour or characteristics of the entire “biosystem” is key.
2) Considering the larger organism, all life as we know it shares one key trait — its processes are ultimately focused on continuing the existence of the life form.
Keeping humans out of the equation so that we can avoid politically and emotionally charged areas, it’s still important to consider the larger organism. There are plenty of examples in the plant and animal world where individuals
cannot or choose not to reproduce themselves (wolves, ants, bees) yet the
overall group is designed to keep the species going.
There are also many examples of self-sacrifice and altruistic behavior in animal groups, but these behaviors are also tied to the overall survival of the species.
In multicelled creatures the role of individual skin cells or liver cells is not necessarily reproduction, but the overall organism coordinates the behavior of the subunits to keep the larger creature going.
Another related problem is how to decide if some piece of matter contains a certain specified organism.
This seems to be a rather trivial exercise once the organism is defined e.g. as a machine that executes some program. However, let’s now consider the fact that information is conserved according to the laws of physics. The present state of the universe is related to the state the early universe was in via a unitary transformation. So, in principle, one can say that we exist also in the early universe (but subjectively experiencing the present state of the universe, of course ).