"R" == Rich  <someone@somewhere.com> writes:

R> You have the arrow pointing the wrong way, either you have evidence
R> of ET, and hence a reason behind your 'reasonable expectation',

Of course, if we had evidence of ET, there'd be no point in having an
argument about whether looking for ET was justified.

I presume you mean, Is there any reasonable expectation that ET might
exist, thereby justifying a search?

R> or you do not. So if you have some positive evidence, feel free to
R> post it, or post any other evidence that your expectation is
R> reasonable.  So far all I've seen is emotional arguments and
R> logical arguments, and of course, the ever-present belief argument.

We know planets are widespread.  More than 5% or 10% of solar-type
stars have Jupiter-mass planets.  Serious selection biases against
finding lower-mass planets, but from the current census it appears
that there are more lower-mass planets than Jupiter-mass planets.

We know of at least two Earth-mass extrasolar planets.

Ergo, it is reasonable to expect that Earth-mass planets are
widespread.

A number of organic molecules, some quite complicated, have been found
in interstellar space and comets and are expected on other solar
system bodies (notably Titan).

Earth is 4.5 billion years old.  The earliest microfossils appear to
be about 3.5 billion years old, and there is geochemical evidence
suggesting that life was present 3.8 billion years ago.  

While we admittedly do not understand the origin of life, one
reasonable (and fairly widely accepted) interpretation of these data
is that life can originate easily, even under potentially quite harsh
conditions.

We do not know if intelligent life or transmitting civilizations
can/will develop once life has originated.  That's the point behind SETI.

-- 
Lt. Lazio, HTML police   | e-mail: jlazio@patriot.net
No means no, stop rape.  | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html