If we knew what we were in previous lives, that knowledge could interfere with our current life. There is a paradigmatic case recorded by Dr. Ian Masterson in his famous study on probable reincarnation cases. William Monroe III was walking through Central Park one day when he had the clear perception that he had trodden that land before it was sold to the Dutch colonists for a ridiculous sum. William Monroe III (who had no clear idea about what he was in his current life, although William Monroe II believed him to be a gastropod), from then on became convinced that in a previous life he had been a Native American. Furthermore, he was sure that he had participated directly in the purchase sell of those lands. Then, he went up on top of the Empire State Building and, looking around, he realized that such deal had been a total scam. Manhattan was worth about twice what the Dutch paid for it (US$24 and a pair of Dutch clogs). According to his calculations, the Chrysler Building alone was worth at least seven pairs of Dutch clogs. Outraged, he ran to the city council to step back from the deal before the claim deadline expired. He demanded from the mayor that the island be returned to him, and he in turn returned the $24 and the Dutch clogs. Of course, he was taken for a crackpot and, although some tabloid TV shows echoed his claim, few New Yorkers went along with packing their belongings and leaving the city within 24 hours as he had required.
