In November 1919, shortly after Albert Einstein's theory of relativity was confirmed by observations made from the island of Principe during an eclipse (light was shown to have been bent by gravity as Einstein had predicted), Sir Arthur Eddington, an early advocate of relativity, was approached by Ludwig Silberstein at a joint meeting of the Royal Society and the Royal Astronomical Society.

"Professor Eddington," Silberstein declared, "you must be one of three persons in the world who understands general relativity." When Eddington demurred, Silberstein continued: "Don't be modest, Eddington."

"On the contrary," Eddington replied. "I am trying to think who the third person is!"

[Eddington was once asked how many people could really understand his theory of the expanding universe. He paused for a moment before replying: "Perhaps seven."]