Somebody once said to me that he was sure that God existed. When I asked why, he explained that it was because he believed in God, and so the existence of God was true for him. As a scientist I reject such reasoning; or at least his concept of what it means for something to exist is fundamentally different to mine.
Today, however, I came across the following observation, which is the opposite to the above argument: Financial markets are only efficient if somebody does not believe that they are.