The explanation of the operation /mechanics of an engine allthough admirable,does not alter the fact that it is useless without the fuel the components of which existed before the engine was ever dreamed of. There is nothing mystical about that.
Likewise the functioning of a body which can be expertly explained, is useless without the fuel which also existed before the body existed, there is nothing mystical about that.
Both examples would not now exist without the processes( evolutionary) which produced an oxygen rich environment.
We could if we wish call spirit oxygen or nitrogen or take your pick, but evolutionary language, sounds, words definitions have brought us to the vocabulary we use.
One such word is spirit,,.what do we do with it consign it to the bin/trash never to be used again?
The O, P. chose to ask of those interested enough to proffer their opinions/explanations of what it is without specifying which definition .
My response was from the outset in human terms( nothing mystical ) that it IS simply the air we breath basically what has given us life, keeps us alive, existed before us,(given that other factors also contributed)but as in ancient cultures air we breathe was seen as spirit(nothing mystical unless you choose for whatever reason you make it mystical supernatural ).
The other definition as a descriptive for human traits as alluded to is also what spirit IS in our vocabulary
Spirit as gods, angels, demons and supernatural force, although so defined is as I have said hogwash imo.
Viv ...I said the act of breathing is NOT SPIRIT you misread.
Best wishes.