In 1975 James Randi offered $10,000 (later increased to $1,000,000) to any person who could perform a paranormal feat (including any demonstration of psychic power) in his presence under controlled conditions. (This offer was still open as of the printing of this edition, but certain restrictions were placed in effect as of September 1, 2015.) Since that time, hundreds of people have tried to win the prize, but not a single one has succeeded. Does the failure of such people to collect the prize constitute a fair test of the existence of psychic phenomena?

During the past hundred years, thousands of experiments have been performed to demonstrate unequivocally the existence of psychic phenomena, but thus far none has succeeded. To account for the negative results of some of these experiments, Gertrude Schmeidler invented the "sheep-goat" hypothesis, according to which such experiments are influenced by the attitudes of the experimenters. If the experimenters doubt the existence of psychic phenomena, then the experiment is doomed to failure; but if the experimenters are believers, then the experiment will succeed. Is this hypothesis believable? Does it qualify as an ad hoc modification? How might the attitude of the experimenters affect the outcome of the experiment? Could James Randi's presence account for the inability of performers such as Uri Geller to demonstrate their alleged psychic powers? (G. R. Schmeidler, "Separating the Sheep from the Goats," Journal of the American Society for Psychical Research, 39, no. 1 [1945], 47-50.)