More obscure analogies

[Rob Seaman]

On Jun 6, 2005, at 4:28 PM, Bernard Vatant wrote:

> Cool. I love recursive definitions :)  That one is certainly  
> operational - fit for everyday life affairs. But to provide a  
> scientific definition of human kind as a species, forget it, it's  
> easily broken ...

Actually, I think it is fairly close to what would be regarded as the  
current "scientific definition".  More to the point, it isn't clear  
that "everyday" and "scientific" definitions are, or should be,  
different.  Certainly complexity of expression doesn't equal depth of  
understanding.

> I would be happy to trace my ancestors back to, say, 200 millions  
> years ago, and meet them one by one - just to check the humanity  
> breaking point

But evolution contains the context for addressing these issues, too.   
Our most recent common ancestor with chimpanzees was something like  
seven million years ago. See Richard Dawkins' fabulous "Ancestor's  
Tale".

> ... Not to mention my descendents 200 millions years from now

It may indeed be impossible to describe entities that do not yet exist.

> "When in Earth history did humanity begin?" is a question exactly  
> similar to : "When in Universe history did galaxies appear"?

Well, no.  The underlying "equations of state", if you will, are very  
different.  Galactic evolution is evolution in name only.  You've  
already pointed out the chicken and egg problem for biological  
entities.  Life is a rich binary tree of cousins - many branch  
points, many "buds".  Galaxies were always galaxies.  Before that,  
they were only proto-galaxies.  Our non-human ancestors had their own  
identities - they were never "proto-humans".  One would be more  
correct to compare the first galaxies to the appearance of the first  
eukaryotic cell, perhaps - but even then the complexity of the web of  
life is both greater (in variety) and less (in the knowledge  
compression of evolutionary genetics) than that of "mere" astronomy.

> a species is still something very difficult to define, and hard to  
> observe. I won't elaborate on the details here, but I had an  
> interesting breakfast with a botanist a while ago, which definitely  
> destroyed in my mind the notion of any "objective" definition of  
> what a species is ...

It may not be surprising that the taxonomy of Linnaeus may need  
tweeking after 250 years.  The notion of a species is quite different  
- perhaps nonexistent - for entities that reproduce asexually.  See  
http://ucjeps.berkeley.edu/map2.html.

> "Undecidable" was not adequate a word, I agree, if you read it in  
> its strict logical sense. I mean you can chose any arbitrary limit,  
> like geographs do for the classification of cities, by setting  
> arbitrary population thresholds ...

But "city" is an arbitrary notion.  "Human" and "Galaxy" have  
intrinsic meaning, no matter how difficult to pin down.  My original  
comments were specifically about the word "event".  An event is not  
only an arbitrary notion - it is a whole constellation of arbitrary  
notions.

Rob Seaman
NOAO
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