[quote=So Boerd]Suppose you and I are in a plane crash on a desert island. I am paralyzed, a quadriplegic and to prevent you claiming the need for human company, unable to speak, while you are unscathed. Why should you try to keep me alive?[/quote]

In that case, it would be logical to assume you'd want to die... This is normally where people turn off the life support for whoever is in a coma.
And sadly you being unable to speak would make communication difficult. How I would address the situation is finding some other means of communicating with you, blinking, grunts etc for yes and no questions.Then try to understand if you desire to live.

If you did, I'd work my hardest to keep us both alive, if you didn't I'd kill you in as painless a manner possible.

Also, [url=https://www.youtube.com/watch?v=e-zaO-hUYag]the person doesn't need to speak to provide human company[/url]

[quote=So Boerd]Also, the first society to acknowledge human rights did so with no evidence. Were they irrational?[/quote]

That would of been theory and testing, they have an idea and they have reason to think it would work like Brovo described.
That first time would of been their test/experiment to see if it does in fact work or not, and as we see that test turned out to give positive results.