Neville Woolf: I'm going to go back to Schrodinger and say that life has to involve at least one process whereby the free energy of the environment is used to harvest more free energy and that automatically creates certain things that we have associated with life. And those things are that life reaches an ecological limit, and that because of that ecological limit, anything else that comes along is liable to latch onto life to try and find sustenance from it either as a symbiotic form or as a predator.

These kinds of things therefore are absolutely fundamental to the basic thing that Schrodinger brought up, the thermodynamics, and it's after that that we have our disagreements as to which ways that thermodynamics can be accepted as part of life.

Steve Benner: I'm going to join Carol Cleland and talk about the futility of making definitions. But I don't quite agree with her in one respect. Because I think that definitions and the attempt to make them are useful because they tell you something about the people who are attempting to make the definitions.

When you say you believe that carbon is essential for life, and when you believe that this rules out silicon as being essential to life, then you tend to come up with the argument that, because silicon is an order of magnitude lower in concentration in the galaxy than carbon, silicon life is not possible. In point of fact, I happen to believe that molybdenum is essential for life, and molybdenum is much more scarce than either silicon or carbon. Boron might also be essential to life, and that is still less abundant. So I happen to think that that's not a good argument against silicon life.

But because we're familiar with carbon life, we would tend to define life as requiring carbon. So definitions are useful, not because they understand or deliver information about the underlying concept, but because they help us understand ourselves.

Pam Conrad: Well, when you get the piece of paper that says you're a scientist and you sign up to use the scientific method, you have to agree to use measurable parameters. And so if you can't define the thing that you're measuring so that you can parameterize it, it's no longer a measurement, and thus it's not science. So the problem here is, while I agree with you that definitions are problematic, it's even more problematic to do science without some kind of measurement, because it's not science and they take away the piece of paper.

So one approach that we might be able to agree on is that some things you can know by the scientific method, and some things we may not be able know by the scientific method. So if we can at least inventory those parameters that we can agree upon and get a starting point, at least we can decide on an experiment that we could pursue, even if it's a thought experiment in the context of this discussion.

Carol Cleland: We just increased our degree of disagreement, particularly about the scientific method. I'm going to try not to launch into discussion of the scientific method, which is extremely problematic. There's a mythology about the scientific method having to do with falsificationism and inductivism, and all of it is logically deeply flawed and historically inaccurate. But I won't go there.

I do want to say something with regard to the idea that you must define 'life' in order to run an experiment. I just think that's not true. People like Steve Benner have been running experiments and engaging in a lot of interesting theoretical thought about the possibility of designing nucleic acids with different nucleotide bases, so I don't think we need a definition to run experiments. In fact, I think a definition is going to act as a blinker. It could actually guide you along lines that make it impossible to recognize strange forms of life if in fact you encounter them. So I think that is self-defeating.

Pascale Ehrenfreund: When I teach my students the basic principles of life -- including self-support, reproduction and interaction with the environment -- they always say that, well, computers can do similar things now and certainly in the future.

Everybody has his own basic principles for life. But I think there are some certain rules of chemistry which one should obey. Carbon is just an incredible element which can form three dimensional structures, and there is no other element which can do this. Actually, that's not completely true -- I read in a paper which Dirk Schulz-Makuch gave me yesterday that silanes can form some kind of large aggregates. But still, that happens only under certain conditions. I think that our carbon-based life is very powerful. The carbon abundance in space argues that this is probably also possible in other environments. I think we should rely on that argument.

Pam Conrad: I think there are two ways you could look at what life is in terms of whether we can agree or not agree. One way is to look at life by what it is, what it's made out of, what we think its properties are. Another way we can look at life is by what it does, by its function. And the problem with looking at life as a function or as some processes, is that once you define life as some stuff that happens through time, you've got to come up with a sampling rate and all sorts of definitions of the length of experiment, and then we're back to defining things again.

But I do think that it is possible to come to some level of consensus with regard to certain characteristics of life. The problem is that we don't agree in toto on all of the characteristics, whether we're talking about life in terms of how we might detect it, or life in terms of how we might make it.

Steve Benner: I agree, but the question that I think we, as well as the public, finds most interesting is whether we have a chance of encountering, in a trek to the stars, life forms that behave like we do, where the fundamental chemistry is based on a polymer of silicon, for example, or living in a solvent other than water.

If you go to the astrobiology web page on silicon life, you'll find it to be a fountain of misinformation. They tell you, for example, that there are no chiral silicon compounds. Of course there are chiral silicon compounds. There are chains of silicon up to 30 or 35 -- they are very interesting; they are studied by chemists. These compounds carry nitrogen, they carry sulfur. In fact, carbon-based life is not really carbon-based life either, it's carbon-scaffolded life, with the oxygen, nitrogen, and phosphorus playing absolutely critical roles.

Now, the argument is going to be more subtle. Maybe silicon is not as easy to generate in prebiotic conditions as carbon-based life. I think that's probably a true statement. Obviously the affinity of silicon for oxygen is going to be a problem when you try to assemble silicon-based life.

But ultimately, we want to know whether you could conceive of life like what we know on Earth, something that is indisputably life, based on something that's quite unusual, at least not carbon at its core, although almost all silicon compounds contain carbon as well, and water not at its core. I think that's the ultimate question for the weird life people.

Neville Woolf: (holds up his laptop computer) It depends on what kind of life you're asking for if you want silicon life. The question is whether you insist that life has to be a chemical phenomenon, or whether you're willing to allow that other physical processes could indeed form life.

I think we can see from the vacuum cleaner that goes around on its own and sweeps where it needs to, that indeed it is possible to have many of these behaviors that we consider to be associated with chemical life also in other principles. And in fact, following the Linnaean system, I'd like to suggest that there's a higher level than Peter Ward suggested, and that it depends on the principles on which the life is built, therefore they should be called "Principalities."

+ Part I of this debate: "Launching the Alien Debates"
+ Part II: "The Dialectic Game"
+ Part III: "What is Life?"
+ Part IV: "The Basic Rules of the Universe"
Beyond Sol

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