To make things seem more wonderful than they are, to possess knowledge and exhibit power beyond the ken of the multitude, has exercised a fascination upon the human mind in all its stages of development. The primitive conjuring of the ancient priest or of the savage medicine-man, the long tradition of Oriental legerdemain, and the stage performances of the modern prestidigitateur are all connected with deep-seated human instincts. It has even been suggested[3] that the mimicry and death-feigning instincts of animals, though essentially biological in type, are yet related to the psychological instincts of deception which make their first clear appearance in the higher animals and assume a distinctive position in the psychological equipment of childhood. The conjuring tricks or paradoxes which apparently contradict or rise superior to ordinary experience, furnish the most various types of illustration of the psychology of deception. Whether presented as miracle by priest or by thaumaturgist or by expounder of the black art, or presented as proof of spirit agency by the modern spiritualistic medium or his less pretentious predecessors, or by the stage performer for entertainment, the analysis of what was actually done, and the accounts of what the spectators saw or believed that they saw, illuminate with striking brilliancy the _modus operandi_ of the processes whereby appearance takes the semblance of reality and observation is shrewdly led astray. The conjurer thus becomes a suggester and an actor, not a mere exhibitor of his manipulative skill. As our present purpose is to investigate the nature of real deception, of the formation of false beliefs which may in turn lead to unwise action, it will be well to note that even such elementary forms of sense-deceptions as those just noted may fall under this head. No one allowed the use of his eyes will ever believe that the pea held between the crossed fingers is really double, but children often think that a spoon half immersed in water is really bent. Primitive peoples believe that the moon really grows smaller as it rises 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 of a confidence in its existence.