What is spirit?

In a two or more posts, I've talked a little bit about "spirit".  If you're a skeptic and you've been reading this, you probably think, that's nonsense, there's no such thing as spirit, it doesn't exist.  Probably not, but then again, there isn't any such thing as energy either, at least not in the sense that you can measure it directly.  It is a derived quantity: it always has to be calculated from two or more other quantities.  Nonetheless it's still an extremely useful quantity in physics.

In particular, I wanted to clarify some statements I made in the post called, My Dream. I thought about simply writing an addendum, but why not create a brand new post?  In that post I talk about negative spirit flowing out of Washington D.C. through a network of trails leading, ultimately, to long-distance scenic trails like the Appalachian Trail.  How does that work?  What is the mechanism behind this?  Simple.  The people walking (or cycling) towards the city are smiling.  Those moving away are not.

I experienced this directly.  I picked up two hikers from the A.T. and let them sleep over in my apartment.  The next day, we made arrangements to meet after I had finished work and they had finished sight-seeing.  Later, I began to worry that I would having trouble finding them since we had agreed to meet at one of the busiest subway stations during rush hour.  It turns out they were easy to spot: they were the only ones smiling.