He did so because he had long disagreed with a lot of the most important and influential physicists of his time, about the interpretation of that area of physics known as quantum physics that deals with the behaviour of objects in the microphysical, subatomic, world. Many of these physicists were committed to an interpretation from which it follows that nothing - the moon included - exists unless it is being observed. Einstein wanted to know whether Pais was on his side or theirs.