The Nation Debt in Three Lists of Six Items

There are lots of very smart people who tell us we don't have to worry about the national debt, despite the fact that it recently passed $20 trillion. Is this true, are we over-reacting? Or is this the classic case of deciding that because something is not a problem now that it will never be a problem? The US can borrow at historically low rates, there are countries worse than us, but what is the endgame? How long can we keep this up?