Good question. As far as I know the best way to find out is to look up the word translated "spirit" and see how it is used in other writings. And then find out what that word meant in the culture and time it was written.
I've heard the idea described as we are the only creatures with the ability to commune with God. But, contrary to what some women have told me, I'm not really a pig, so I can't tell you if pigs can commune with God. Then again, this idea may be way off base.