Paul McCartney wrote this song. It's about a man who is considered a fool by others, but whose foolish demeanor is actually an indication of wisdom.
An event which prompted this song happened when Paul was walking his dog, Martha, on Primrose Hill one morning. As he watched the sun rise, he noticed that Martha was missing. Paul turned around to look for his dog, and there a man stood, who appeared on the hill without making a sound. The gentleman was dressed respectably, in a belted raincoat. Paul knew this man had not been there seconds earlier as he had looked in that direction for Martha. Paul and the stranger exchanged a greeting, and this man then spoke of what a beautiful view it was from the top of this hill that overlooked London. Within a few seconds, Paul looked around again, and the man was gone. He had vanished as he had appeared. A friend of McCartney's, Alistair Taylor, was present with Paul during this strange incident, and wrote of this event in his book, Yesterday.
Both Paul and Alistair could not imagine what happened to this man. He had seemed to vanish in thin air. The nearest trees for cover were too far to reach by walking or running in a few seconds, and the crest of the hill was too far as well to reach in that short time. What made the experience even more mysterious, was that just before this man first appeared, Paul and Alistair were speaking to each other of the beauty they observed of the view towards London and the existence of God. Once back home, they spent the morning discussing what had happened, trying to make some sense of it. They both agreed that this was something others were infer occurred as a result of an "acid trip," but they both swore they had not taken or used any drugs. When Paul filmed the sequence for this song in the film, it shows him on a hilltop overlooking the town of Nice. (thanks, Gavin - Hampden, MA)