Critics of the experiments include psychologists David Marks and Richard Kammann, who published a description of how Geller could have cheated in an informal test of his so-called psychic powers in 1977. Their 1978 article in "Nature" and 1980 book "The Psychology of the Psychic" (2nd ed. 2000) described how a normal explanation was possible for Geller's alleged psychic powers. Marks and Kammann found evidence that while at SRI, Geller was allowed to peek through a hole in the laboratory wall separating him from the drawings he was being invited to reproduce. These drawings were placed on a wall opposite the peephole which the investigators Targ and Puthoff had stuffed with cotton gauze. In addition to this error, the investigators had also allowed Geller access to a two-way intercom, enabling him to listen to the investigators' conversation during the times when they were choosing and/or displaying the target drawings. These basic errors indicate the great importance of ensuring that psychologists, magicians, or other people with an in-depth knowledge of perception, who are trained in methods for blocking sensory cues, be present during the testing of psychics. Marks, after evaluating the experiments, wrote that none of Geller's paranormal claims had been demonstrated in scientifically controlled conditions, concluding that "Geller has no psychic ability whatsoever. However, I believe him to be a very clever, well-practiced magician."