Sgt. Pepper
All you need is love.
Let me see if I can put it simply: THERE IS A WAY TO KNOW SOMETHING ABOUT THAT WHICH YOU CANNOT PERCEIVE WITH YOUR 5 SENSES.
It is called religion which comes from revelations from God through Messengers.
Then there's the sixth sense (Extrasensory perception) and people like me who have it. However, I don't attribute my mediumship to a religion, revelations from God, or any gods. And the only messengers I'll accept as genuine and trustworthy are my devoted spirit guides. I know other mediums who choose to combine their religion and belief in God (or gods) with their mediumship, but I choose not to. My spirit guides have always been sufficient enough for me.
Just because I cannot demonstrate it that does not mean it does not exist. That is completely illogical.
A demonstration is just what you want. It does not make the soul exist.
Conversely, lack of a demonstration does not make the soul nonexistent.
I don't believe that you can effectively demonstrate a human soul to someone who doesn't want to believe and has already made up their mind not to believe. As I've always said, "Seeing is believing," and time and time again, I've witnessed diehard skeptics become true believers after having their own paranormal encounter that they couldn't honestly debunk and explain. The most effective way I've found is to give them a reading that they can't refute.