Let's take the example of the soul and let's "imagine" a way to prove it.
We have two sizeable pools for two people to voluntary drown themselves in, forcing the souls, 2, to separate from the 2 bodies.
Let's "imagine" further that the 2 souls are so friendly and gentle to one another that they are able to follow one another in the forms of 2 souls, human-sized, I guess.
Then some "diverse" interaction between these 2 souls. Then, from this "out-of-body" state the 2 souls decide to retrieve their bodies by "flying" to these 2 bodies lying there lifeless in the 2 pools and "retake up" their respective bodies.
Then, we at the outside of the situation, the friends to these 2 bodies with souls in them, these 2 people, may note they begin to wake in the pools and help them out of the pools.
Later, these 2 people go through (f)MRI scans to confirm the new mental content that also light up "reality" mark in these 2 heads/brains.
I suggest, by the above, that we "now" (by our current imagination here at the blog, Whatiswritten777, reading) have proven the existence of soul/souls, interacting and more.
Proof by inter-subjective/objective facts in 2 people and by clinical death state in these 2 people while "soul-walking". Also, the new (f)MRI scans support our findings.
Some technical details, suggestions:
Time: 20 minutes to "drown" and 20 minutes of "flying time/soul time". Oxygenated pools and possibly (lightly) chlorinated. Water temperature: 22 degrees Celsius.
Pools, 4x4 meters each, in order to hold sufficient oxygen/air.