Well, Barty obviously didn't see it

And no proof is iirefutable. There's such thing as photoshop. Or even social pressure. I know of people who were married for decades and had several kids before they realised they were gay. (I also know of a few perfectly straight girls who have kissed other girls just for the attention...)
But anyway, how many people can rationaly say that there are spirits.
The idea is that you are not your body - you are simply the user of your body.
Again, I doubt that applies to a Death Guardian. Horribly mutilated 9-foot zombie-monster things don't strike me as the religious type...

And even still, it still doesn't mean he would refer to another person as a 'spirit'. Sort of like I believe that, until I meet you in RL, you are simply a name on a computer screen. I'm still not gonna say "Stop wasting my time with talk, Name-on-computer-screen"