What will physical evidence of the not-existence of something (in this case, an intelligent civilization) look like?

To put it differently: show me the evidence that a Purple Teapot did at at one time exist somewhere between the orbit of Jupiter and Saturn. I dare you! Go on! Do it! HA! You can't, can you! Well, then, Mr Smarmy Skeptic! I'll then conclude that its reasonable to believe that .... etc.

Given the raw limits of reason, the empirical/inductive proof of the non-existence of intelligent civilization on mars is not likely to exist, ever. Even in deduction (which is the wrong universe for this sort of thing), the best that you can do is derive the contradiction of some claim. And then that is meaningful only if the system you work in is provably consistent and the premises/axioms you take are reasonable. In fact, incompleteness abounds, and the fact that you can't prove the negation of a claim may not be meaningful.

Probability wise, the argument for the negative case can be very convincing. But some people will spend their lunch money on lottery tickets, so take that as you will.

My point is that Skeptics are met over and over with a snappy raised eyebrow and haughty voice saying: 'Well, you can't prove that NOT Blah. So there, you uppity smartypants! BLAH!'. or, the weaker response: 'Well, you can't prove that NOT Blah. So its a reasonable question as to if Blah or NOT Blah.'

But thats not the way reason works. Empirically verifiable Claims that can't be proven should not be thought of as true or even very likely. Because if they were, you could prove them. Not being able to prove the opposite of a claim has no meaningful bearing on the truth of a claim, because not being able to prove a negation inductively or empirically or deductively is just not very meaningful.

Don't blame me, blame the limits of rationality. If you want to persist in entertaining unproved and improbable hypotheses, then go right ahead. Just don't think of yourself as a rational person while you do it, and certainly don't question the rationality of those who hold those claims as suspect.

Yeti