Other Harvard scientists note that sleep disorders occurring in otherwise normally functioning individuals can produce dream like visions that seem real.

"There is one very obvious explanation for some of these reports, where you experience something strikingly similar to what these people have described, called Isolated Sleep Paralysis," says Associate Professor of Psychology," says Associate professor of Psychology Richard J. McNally, who teaches Psychology 1240, "Abnormal psychology."

This type of sleep disorder occurs in the rapid eye movement (REM) stage of sleep, when the individual is partly awake, McNally says. Typically, the individual cannot move and imagines a looming, ominous shadowy figure.

"If there is someone who believes in aliens flying saucers and what not, they may even more from this [sleep disorder]," McNally says.

Other professors say there is not enough proof for Mack's ideas to be taken seriously. Many skeptics have explained away physical marks-which are often superficial bruises-as simple self-inflicted injuries.

"There is no scientific or strong experimental evidence that people can be sensitive to these external forces," says Assistant Professor of Psychology Michael E. Hasselmo. "There are so many more plausible explanations that there is no reason to talk about [alien abduction] being crue."

Similar stories from people who have no contact with each other prove nothing, McNally says.

"This is not the slightest bit surprising. Our culture has produced these ideas," he says. "If you told me to make up a story of abduction, I'd think of the B-movies from the '50s with aliens that have big eyes and silvery colored skin who fly around in saucers."

Mack Responds

Mack argues that his skeptical coleagues are simply too limited in their thinking. To dismiss an idea because it does not fit in the common notions of reality is unscientific, he says.

"I was raised believing that tall that really exists is the physical world, but now do we decide what really exists?" Mack says. "The one percent in the media and politics decide."

Mack himself comes from a skeptical, agnostic family background, he says. A child psychiatrist, he attended Oberlin College and Harvard Medical School and was trained in psychoanalysis at the Boston Psychoanalytic Institute.

"No one could be more surprised than I do discover people of another universe," he says. "I would come upon something like this only very reluctantly".

He came to accept the possibility of alien life in 1990 after meeting and speaking with Budd Hopkins, a New York artist who has published a bestselling book, Intruders, about alien kidnappings.

Hopkins' interviews with experiencers convinced "Mack that his theories had some validity, and Mack began seeing experiencers in his own practice.

Over the last three years, he has treated about 90 abductees, often using hypnotic regression to bring out their experiences, and 13 of these interviews are the book's case studies.

Mack's theories about aliens are part of a larger system of belief in the importance of the spiritual life, one which stresses the need for environmental healing.

"The UFOs take all elements that are supposed to be part of the spiritual world and impose upon us a phenomenon that does not respect this division of labor [between the spiritual and physical worlds]," Mack says.

The findings in his book are more akin to the spiritual realm, not the physical he says.

"I'm telling something which involves some kind of truth, but it's not something I can prove," Mack says.