Here's my takeaway.
When the boy's finger was cut off, he realised he was merely imitating the truth. Gutei who knew of only one truth, one source, symbolised it with the one finger. The boy realised the meaning of the ONE. There can never be two.
We split our understanding, our self image, our many cultural beliefs and what not....we are lost in the various idea of who we are. Arriving at each other's doorstep with preconceived notions. So in Gutei reflecting the truth of the One, regardless of where one's mental construct is, it will work.
Despite the colour and flavour of our delusion, the truth is ways one. Oii