The existence of parapsychological phenomena and the scientific validity of parapsychological research is disputed by independent evaluators and researchers. In 1988, the U.S. National Academy of Sciences published a report on the subject that concluded that "no scientific justification from research conducted over a period of 130 years for the existence of parapsychological phenomena." No accepted theory of parapsychology currently exists, and many competing and often conflicting models have been advocated by different parapsychologists in an attempt to explain reported paranormal phenomena. Terence Hines in his book "Pseudoscience and the Paranormal" (2003) wrote "Many theories have been proposed by parapsychologists to explain how psi takes place. To skeptics, such theory building seems premature, as the phenomena to be explained by the theories have yet to be demonstrated convincingly." Skeptics such as Antony Flew have cited the lack of such a theory as their reason for rejecting parapsychology.