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prior probability attached to a given hypothesis affects the strength of evidence required to make a rational agent change his or her mind. Suppose, for instance, that in the case of psi we have the following hypotheses:

H 0 = P r e c o g n i t i o n d o e s n o t e x i s t ; H 1 = P r e c o g n i t i o n d o e s e x i s t .

Our personal prior belief in precognition is very low; two reasons for this are outlined below. We accept that each of these reasons can be disputed by those who believe in psi, but this is not the point—we do not mean to disprove psi on logical grounds. Instead, our goal is to indicate why most researchers currently believe psi phenomena are unlikely to exist.3

As a first reason, consider that Bem (in press) acknowledges that there is no mech- anistic theory of precognition (see Price, 1955 for a discussion). This means, for instance, that we have no clue about how precognition could arise in the brain—neither animals nor humans appear to have organs or neurons dedicated to precognition, and it is unclear what electrical or biochemical processes would make precognition possible. Note that precogni- tion conveys a considerable evolutionary advantage (Bem, in press), and one might therefore assume that natural selection would have lead to a world filled with powerful psychics (i.e., people or animals with precognition, clairvoyance, psychokineses, etc.). This is not the case, however (see also Kennedy, 2001). The believer in precognition may object that psychic abilities, unlike all other abilities, are not influenced by natural selection. But the onus is then squarely on the believer in psi to explain why this should be so.

Second, there is no real-life evidence that people can feel the future (e.g., nobody has ever collected the $1,000,000 available for anybody who can demonstrate paranormal performance under controlled conditions4, etc.). To appreciate how unlikely the existence of psi really is, consider the facts that (a) casinos make profit, and (b) casinos feature the game of French roulette. French roulette features 37 numbers, 18 colored black, 18 colored red, and the special number 0. The situation we consider here is where gamblers bet on the color indicated by the roulette ball. Betting on the wrong color results in a loss of your stake, and betting on the right color will double your stake. Because of the special number 0, the house holds a small advantage over the gambler; the probability of the house winning is 19/37.

Consider now the possibility that the gambler could use psi to bet on the color that will shortly come up, that is, the color that will bring great wealth in the immediate future. In this context, even small effects of psi result in substantial payoffs. For instance, suppose a player with psi can anticipate the correct color in 53.1% of cases—the mean percentage correct across participants for the erotic pictures in Bem’s Experiment 1. Assume that this psi-player starts with only 100 euros, and bets 10 euro every time. The gambling stops whenever the psi-player is out of money (in which case the casino wins) or the psi-player has accumulated one million euros. After accounting for the house advantage, what is the probability that the psi-player will win one million euros? This probability, easily calculated from random walk theory (e.g., Feller, 1970, 1971) equals 48.6%. This means that, in this case, the expected profit for a psychic’s night out at the casino equals $485,900. If Bem’s

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This is evident from the fact that psi research is almost never published in the mainstream literature. See http://www.skepdic.com/randi.html for details.

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