I assume that you did not read all the posts so you missed the question, therefore I'll ask the question again.

But first I'll give a little bit of the back ground of "why" I ask the question in the first place.

I have been observing various people debating the existence or nonexistence of extraterrestrial craft and/or their occupants. "They" being the craft and occupants.
There seems to be several view points about the subject, and the debates can become quite intense.

Some of the view points are as follows:

1) "They" do not exist
2) "They" can not exist
3) If "they" do exist somewhere in the far reaches of the universe "they" cannot or have not visited Earth.
4) Perhaps "they" do exist and may have visited Earth
5) "They" do exist and have visited Earth
6) Personal experience has convinced that "they" have or are visiting Earth

Many of the people in the # 5 & 6 category claim that some of the extraterrestrial "space craft" have the shape of a "cigar" or a "saucer" and that these "craft" fly without creating a lot of noise, have no wings, can fly at great speeds and can hover, also they are able to make turns and maneuvers that are considered "impossible" by the aerospace engineering and science community.

The people in the # 1 & 2 category are usually involved somehow in the engineering or science community.

I can understand, at least to some degree, the reasoning of the people in the # 3 to #6 categories, but I do not fully understand the reasoning of the people in the # 1 and 2 categories and that is "why" the question.

The question revolves around the perception of "impossibility" of the existence of such a craft.

I can certainly understand that the engineers and scientist can not comprehend how such craft could be constructed with today's knowledge and technology, but does that make it "impossible" to "ever" construct such devices.

This controversy reminds me of a story I once heard about a "blacksmith" and a "traveling salesman". The blacksmith knew his trade and everything that could be known about iron, (so he thought anyway.) One day a traveling salesman asked him what he thought of the idea that he had heard about, the idea was to construct ships out of iron. The blacksmith considered such an idea impossible and to prove his point he threw a flat iron into a lake saying that "you can see that iron does not float."
I suspect that the engineers and scientists of today maybe in a situation somewhat similar to that of the blacksmith of long ago, that is, they have huge quantity of knowledge about their field of work, but do not understand all the laws of nature in order to construct the mentioned "cigar" or "saucer" shaped craft.

So the question is: At what point in the development of a "cigar" or a "saucer" shaped craft would a scientist or an engineer believe that such craft can be made operational?

A) Would the explanation of the operation be enough?
B) Would the plans, drawings and mathematics be enough?
C) Would the demonstration of the basic operating principles be enough?
D) Would the demonstration of an operational model be enough?
E) Or would it need to be fully operational craft that could be used to transport the scientists and engineers from one place to another?

I hope that this clarifies my viewpoint on the subject matter and the question.

Johannes