Why recycling is such a good idea

I had a message yesterday from an American lady who is coming to Edinburgh next weekend with her husband. He is a professor of Neurology with an interest in David Hume and she wants me to take him for a tour with as many Hume references as possible. David Hume was the shining star in a galaxy of 18th century Scottish philosophers such as Adam Smith, father of Economics and Adam Ferguson, father of Sociology.