What is the nature of belief? What are the conditions under which it arises? and What are the causes for its appearance? The purpose of this book is to answer these questions and to investigate the nature of deception, of the formation of false beliefs leading to erroneous action. "No one allowed the use of his eyes will ever believe that the ball held between the crossed fingers is really double, but children often think that a spoon half immersed in water is really bent. Primitive peoples believed that the moon really grew smaller as it rose above the horizon, and the ancients could count sufficiently upon the ignorance of the people to make use of mirrors and other stage devices for revealing the power of the gods. The ability to correct such errors depends solely upon the possession of certain knowledge, or a confidence in the existence of such knowledge..."