What is Spirit?

It's an interesting side to not: the spirits become viewable by their own will, or they appear only when it's allowed for them to?

Well, I do not refuse for angels to be seen by us, or even be among the people. All I know for certain is that I am a nervous often, I'm not very calm person, so even if a true angel visited me, I couldn't be 100% sure.

I remember I was passing some exams in a seminary, when one student became telling me about one person (I never met) that that person got serious problems with his head, because he said he saw angels. I know that quite similar symptoms can be monitored when a person is suffering by schizophrenia, however I don't trust that another psychological stuff. Never trusted. When that student told me that (I did not realize why for), I felt pity about that guy. He might be a good one person, but the others could make him believe that he was a loony. I guess people need in a positive attention, especially when they are like that guy. I think God works in mysterious ways.

I think that definitions are mostly useless if there are no either similar objects, or there are no need in systemathizing it. Let's say we're going to inventary the items been found (maybe we're archeologists or something), and we put different objects into different categories as vases to the art, shovels to the tools, etc. We can change the principles putting vases into ceramics and shovels into wood-iron composition, but in any case we have to put the spirits somewhere... 
Oh, I know what example we may use - the one in Edgar Allan Poe style: if being a butler (house steward, etc) we want to catalogy a mansion's inventory, and that mansion is inhabited by the ghosts, so to which category we have to put them?
I think that definitions may not help us, but in many reasons it has its purposes. 

Perhaps, it's the shortest, yet a meaningfulness answer in the forum.

So, if "I" vanishes, the spirit vanishes, is that right? What about those persons in coma or when they are unconscious? Is the spirit presented in them at the time?

There's another question appears (asked mostly by the critical realists) what is the difference between "I", "me", and "he/she"? "I am afraid", while "it scares me". Does "I"="me"?

As for me 'me' sounds quite heavy, just like 'me' is an object, a rock, or a gathering of molecules. 'Me' ruins 'I'. 
However, when a person is sleeping it might happen that the other part of him - the one that was the part of his "we" - is awake, and does the cruely, immoral things - exactly as Dr. Hyde did in that famous book. Such a situation might happen.

Do you think people in the past did really invent the concept of soul? In my opinion usually things go like in this: one said another one else about something the one didn't even realize, and so did the another one, and then the third one caught this concept (not realizing about it either), and that endless circulating of rumoring went on... I am rather follower of this version, because to create such a concept is soul is either too simple, or too complex. If it it a simple thing, then, let's say one barbarian told another one that that person was still alive after one's death, and that another barbarian thought that if that person was still alive after one's own death, then something must stay alive... and he called that 'something' a soul. Then the first barbarian agreed about the term or maybe corrected his collegue, and thing went like that. 

If it was a complex concept, then it seems like someone of philosophers invented it. Like Plato. The last one liked to invent different gibberish concepts and stories. So, he can easily put some rare ancient god of something instead of that something that is alive after one's death, and to use it.

Besides there's another story I like not less. Let's say in different cities in Babylon or somewhere in Egypt (actually, I don't know where, because I don't know too much about the ancient times) some priests or the chiefs wanted to take more money (or cows, bulls, goats, etc - I am not familiar of what the currency was being used those times either), and they want not to take it for something daily, but just from the pure air. Actually, the chiefs and priests did nothing indeed, except for watching the people and to utter orders, or some prayers. So, they wanted to collect more wealthy, and for that they decided to deceive foolish ancient hillbillies saying them that after someone dies it lefts something also. And they assured those ones to pay them, because the chiefs or the priests could pray for the souls of those persons. And so on.

I think this last one version is the most true among the others. It's pragmatic, and it explains why some concepts are alive - these concepts are proved to be circulating, because they are being used in economic life of people. That is why today I hear almost nothing about soul, except for some idiomatic phrases where this concept is used for different kinds of expressions. Or in comics - sometimes the artists draw the light behind persons to emphasize the dramatic moment. Or in some animation movies, and so on.