[AntiGuv]

We cannot see well enough to tell one way or the other. In order for us to appear to be alone, we must see the universe in such a way that it appears no one else exists. We cannot see but an infinitesimal fraction of the universe that well. At most, we appear not to have identified anyone else.

[ClearBlueSky]

Standing in the middle of the Sahara you could believe you were the only human on the planet, too. Of course you'd be wrong, but perspective is everything-galactically speaking.

[RightWhale]

So far, we appear to be alone. There is nothing in the Hubble volume that has presented itself as being the work of any form of life that we can see.

[LibWhacker]

I prefer to think of it as a statistics problem... Remember, we can often tell much about an infinite sample space by looking at a very small finite sample. In effect, that's what we're doing with all these different SETI studies, that are either already concluded or still underway.

Now, do we have a "good" sample? That's an entirely different question. I think it can be argued either way; we have looked at thousands of nearby stars and so far, nada. So that sample is definitely telling us something.

I don't think we'll ever answer in the negative the question, "Is there life out there?" For that we must look at every star, planet and moon in the universe. But what we can answer (estimate), perhaps very soon or even right now, is how prevalent life is.