According to dsgrue, observation precludes existence. I challenged that definition with the evolution of sentient beings (sensors) ex-nihilo, from matter.

If that were true, since nothing exists prior to observation, then nothing existed prior to the sentient beings. Yet we know that sentient beings come from matter, which needed to exist prior to the inception of sentient beings, hence his definition failed the concepts of reality we're most familiar with.

Despite the effort and critical thinking, I was called a troll and I'm being pushed out of the thread. Gallileo anyone?

Oh...I see the problem with defining existence in that way.

This is why, when you debate religion, you need to be patient, not come in with an angry mindset, and keep an open mind.

Someone who isn't from the same background as you could be correct, and possibly could be sane and even possibly intelligent.

Let's give each other the benefit of the doubt, and if doubting, there are other means than rage.

I believe this is a lesson learned for everybody.

Tolerance, love, and respect go a long ways to increasing our common understanding of the things around us, without having to compromise our commitment to what we believe is true (on all sides of the debate).

(This coming from the guy who has had repeated offenses for flaming. Know that there is hope.)

As a wise person once told me, if you can't stand hearing ideas that challenge yours, get out of the boardroom.

From what I gathered dsgrue3 definition was that for something to exist it needed to be observed by another, I am no expert but  his definition  ,for my part, was quite wrong as the term "known to exist thus far" is often used, which I always thought meant something could exist without someone knowing its there.

Anyway that's my two cents.

Yes, that's exactly the point. Granted, there's some gray area in between, such as whether we can truly claim there are no unicorns or invisible green men in the palm of my hand....which can be argued against by appeal to Ockam's razor, which in turn then has to be justified.

A good example is the discussion of alien life. I actually think Dawkin's mentioned this in the God Delusion....the question being whether we can infer the existence of aliens without observation of them given the vastness of the universe, the chances of life developing, etc.

Edit: Discussions about  the multiverse are also a good example.

Well, naturally you cannot affirm existence by itself, thus it need be observed to exist. Circular logic doesn't work. 

We have observed fossils of dinosaurs, so their existence is affirmed. We have oberved atoms and matter at present, confirming their existence. 

I think this clarifies the definition provided.

We cannot say with certainty that aliens exist, but we can say with certainty that the probability of them existing is vastly superior to the probability of them not existing.

Maybe...maybe not. We don't really know the probabilities involved. Anyway, that wasn't really my point. My point was that we can actually have meaningful discussions about non-observed entities.

That is a good point. But we can't say, in the converse, that until the fossils were found, that dinosaurs didn't exist. That's the flip-side of it.

I think you might be confusing confirming something exist through evidence and something existing in of itself. For simplicity sake let me use myself as an example:

If there were no internet and there was no profile on vgchartz called cyberninja45, you (dsgrue3) would not be able to confirm that there actually is a person that is capable of typing this response, but your inability to confirm my existence (through the internet) does not negate my existence for 20 something years on the earth.

In other words I still existed whether you had evidence or not.

Hey, it is called having an opinion, you shouldn't feel bad when you fail at convincing a religious person.