Tim Tyler  wrote in news:bogi65$1paq$1
{at}darwin.ediacara.org:

> Anthony Cerrato  wrote or quoted:
> 
>> Yep! There really is no way to define living things to
>> include only what we want them to include IMO. To me, this
>> means there simply is a major flaw in the concept
>> "living"--i.e., there is no such definable category. I think
>> that anything sufficiently "complex" (ha! define that!)
>> having dynamic qualities of motion (internal and/or external
>> energy processing systems) should be considered no different
>> than life. "Life" is a meaningless term, physically and
>> philosophically, no matter how "special" we want to make
>> it--there is nothing uniquely "noble" about it, as much as
>> our egos make us pretend it is. By most old definitions,
>> _clouds_ are also complex "life!" There are many other
>> dynamic systems which are also. Why waste time trying to
>> create useless definitions? [...]
> 
> Well - there really is something called life - and most
> people know it when they see it.
> 
> Definitions of life are bound to have some problems with
> borderline cases - but that's true of many definitions -
> and sometimes it does help to say what you mean by the
> terms you are using.
> 
> A definition will come in handy when it comes to
> evaluating claims to have made living organisms ;-)

Tim says almost exactly what I was going to say. Of course the argument 
that "I can't define it but I know it when I see it" has been famously 
applied to pornography, and it makes me uncomfortable -  if I know life 
when I see it, it seems that I should be able to give some definition to 
it.On the other hand, since definitions are made of words, which have 
developed as symbols of everyday utility, there may be no reason to 
suppose that there would be words to additionally define what is 
otherwise an obvious distinction. For this very reason there aren't 
additional words to define the color blue, because it is an observational 
artifact. 

Now we can go on to say that while there may be no words to define life, 
there should be some measurement we can make or test we can perform. In 
practice, going back to Pasteur, the test we perform is that of 
reproduction. There are some obvious problems with this test - there are 
organisms that are clearly "alive" that can no longer reproduce, and 
there are viruses and prions that are not clearly "alive" that can  
reproduce, and many organisms require special conditions to reproduce - 
but this may still be the best general definition. 

Yours,

Bill Morse
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