Twelve reasons we haven't found aliens

[Yazata]

For those who need ''evidence''...what would suffice in order for you to believe in the possibility of alien life? (extraterrestrial)

Well, our example-of-one, our Earth, suggests that it's indeed possible for life to arise (somehow).

Then given the incredible scale of the universe, billions of stars in each galaxy, and billions of galaxies, suggests (but doesn't prove) that life might exist elsewhere too. We have no idea how widespread the "abiogenesis" of life may or may not be. Maybe life appears anywhere the conditions are right. Or maybe it's exceedingly rare. (That would be my own guess, but I have no way of knowing.)

That obviously doesn't constitute an argument for the existence of alien life, but I personally think that it's a persuasive argument for the possibility of the existence of alien life.

So... I'm already a believer in the possibility of alien life.

[Zinjanthropos]

If life begins in a watery environment then it may be that this is where it stays. Transitioning to land may be a very rare occurrence for life containing planets. Thus it might be highly unlikely technology advances underwater and if it does then it would be so vastly different from ours that we may never detect it. It could be that most of the life in the universe is contained in oceans beneath a shell of ice, with little need to contact us or anyone else. 

That’s why I loved the movie The Abyss.  A super intelligent underwater civilization, complete with a technology so vastly different from ours that we never even suspected they were there.

Yes, I agree. At least here in our Solar System, water oceans under shells of ice might be more common than Earth like conditions. If that's true in the rest of the universe, it's conceivable that most life lives in conditions more like a moon of Jupiter or Saturn.

They may have no idea that the rest of the universe is even out there.

Should be on the list. Do you think life could also thrive more within a planet than say on surface, exposed to just about everything? Not saying it would develop intelligence but an intelligence could certainly find a way to live beneath the surface, I would think. Should be on list also, IMHO.