The great philosopher Ludwig Wittgenstein was once sitting in a park having a philosophical discussion with a friend when his friend, quite animated, stood up and said loudly, “That is a tree! I know for a fact that that is a tree!”

An awkward pause ensued as the two men realized that passersby had stopped and were now staring at them. Wittgenstein, thinking quickly, turned to the people and said, “Do not worry, this fellow is not insane… we are merely doing philosophy.”