Note: This is a reply I posted to RAF in another thread, arguing what assumptions, if any, one should make about the existence of alien life. As my time is somewhat limited, I also meant for it to serve as a response to some of the arguments advanced by Selfsim and noclevername. I copied my response to here so we could continue the discussion without derailing the other thread. I did not copy other posts because without moderator privileges it would be too tedious to do so.
First, the Nessie or Sasquatch question. I was using those as examples of an absurdity. Namely, that no one here is advocating any of the absurd claims that you see from cryptozoologists, but I don't see a real distinction between how some react to cryptozoology claims and how they react to claims that extraterrestrial life exists.
Personally, I am inclined to believe that life in the universe is relatively rare, and intelligent life even more rare. Please note, I make a very strong distinction between belief and knowledge. But, I am unwilling to discount the possibility of other life in the universe simply because of what we do know about life on earth. We know that life on earth arose almost immediately after the Late Heavy Bombardment, which is probably just about as early as it was able to exist. Absent some other mechanism for creation, that leads me to believe that it is not difficult for life to, at minimum, get started. Some form of life probably started many times on the ancient earth. It's unlikely that mother nature got it right on the first try.
We also know that the entire universe, including where both of us are sitting, started in the same initial conditions. So, unless Earth is really special, it is mathematically unlikely that no other life exists. Of course, as you would say, the mathematical likelihood of something happening does not constitute evidence. And I agree with that, which brings me to my final point.
No one is assuming that life exists. The exact statements were all of the variety of "I wouldn't assume life does not exist." You are equating "I assume something" with "I do not assume not-something" when they are not logically equivalent. To make any kind of assumption, a reasonable person requires evidence to back up that assumption. We almost universally reject the notion that sasquatch exists because we have explored almost every square foot of the land surface of the earth and never found any evidence to confirm his existence, yet we have found evidence of all other types of life spanning billions of years. Therefore, it is absurd to assume Sasquatch exists because the overwhelming evidence says he does not.
In the case of Sasquatch, we have clear and overwhelming evidence of absence. In the case of alien life, what we have is absence of evidence. Again, evidence of absence and absence of evidence are not equivalent. A doctor who thoroughly examines a patient and finds no evidence of cancer can safely say, "I assume there is no cancer." But a doctor who has examined only the tip of a patient's nose can only say, "I cannot assume that there is not cancer." No other statement about whether or not the patient has cancer can be logically made.
In the case of the question of existence of alien life, the absence of evidence overwhelms the evidence of absence. We have only closely examined a vanishingly small part of even our own solar system. We don't even know how to look for extraterrestrial life. We don't know if there are microbes in the middle cloud layers of Venus, the supposed oceans of Europa, or the hydrocarbon lakes on Titan. We know so little about them that it is logically impossible to assume that life does not exist. The only thing we can do is to not assume that life does not exist. If you are not assuming something, it means that you are assuming nothing. Taking any other position with so little information available cannot be logically supported.