The beauty of definitions is that they are arbitrary, but a necessary starting point in this case to begin understanding why someone may believe in the existence of the spirit/soul. While definitions may vary widely, each one can be put forth for scrutiny.
Using the instruments of physical science to try to observe metaphysical things is like trying to see radio waves with the human eye. The capability simply does not exist.
Sure. My interest, however, is in how the spirit/soul interacts or supervenes on the physical. This seems to be a common notion put forth by those whom embrace this concept. There should be some measurable effect if there is a connection. In what way can this be quantified? It seems to me that in the past there was a now debunked study that proclaimed that the human body loses X ounces upon death failing to account for various bodily fluids which seep out upon expiring. This was said to be the soul.
I see no efforts these days to produce any semblance of support for these concepts. Lost and relegated to the legends of old, yet still very much in the mind's of the public as a necessary precondition of X religious belief.
But what if the effect of spirit / the soul is simply consciousness? Consciousness must be manifested in the physical world through a physical mediuum even if it ultimately resides in / originates from a metaphysical entity. Therefore if spirit is the origin of consciousness it will still appear to us as if consciousness arises from biochemical neural processes, because the brain is either the seat of physical consciousness or the conduit through which spiritual consciousness manifests. The existence of spirit does not require its effect on the physical world to be identifiable and distinct from phenomenon we associate with physical mechanisms. Spirit, therefore, may be hidden in plain sight. It may be plainly manifested in all life by virtue of life existing, yet incapable of being seen because it cannot be isolated from life, which we view as being a purely physical phenomenon.
On somewhat of a flip side, my sister sees dead people, so at least from her perspective spirit is directly perceptable. Whether she's dellusional or gifted will depend on your point of view. She's a lawyer, highly intelligent, and has had a long and successful career, which tends to suggest she's not batshit insane. But then she recently moved to Australia, which somewhat puts her sanity in doubt. But having that experience of dead people does not necessarily anwer your questio of what is spirit and how does it act in the world? In particular because my sister tells all her visitors to go away, so she doesn't seek to facilitate the world of spirit having direct action in the physical world. And anyone who she didn't know in life can't break through to her, though when she was a young girl they could and she had to work hard to block unknowns out? End of the day these personal experiences only have meaning to the individuals concerned and they are believed or not believed based on the theological biases that existed with the person before they hear of such thing. If anyone tries to tell my sister that spirit and the soul don't exist and that individuality ceases at the point of physical death she will firmly but politely disagree, but she has no better idea than you or I how these realities act within people and on the world while people are alive.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."