SANTA MONICA, Calif. -- Richard H. Popkin, a retired professor of philosophy at the University of California, Los Angeles, who became a specialist on skepticism and its history through the centuries, died here Thursday, his family said. He was 81 and had emphysema, which caused him to use a wheelchair.
Dr. Popkin, despite limited vision, had been working on a book about the 16th-century Rabbi Isaac of Troki in Russia, said his son, Jeremy, of Lexington, Ky.
As an author, Dr. Popkin published his most durable book, ''The History of Skepticism From Erasmus to Spinoza," in 1960, and continued updating it through a 2003 edition.
He attracted mainstream readers with such books as his 1966 ''The Second Oswald: The Case for a Conspiracy Theory," about the assassination of John F. Kennedy. In the book, Dr. Popkin strongly challenged the finding of the Warren Commission that Lee Harvey Oswald had acted alone in fatally shooting the president during a 1963 motorcade in Dallas.
Dr. Popkin also earned widespread attention for the 1998 book he co-wrote with David S. Katz, ''Messianic Revolution," about radical religious politics.
An internationally known authority on the interaction throughout history of Jewish and Christian philosophy and theology, Dr. Popkin examined millennial clues from the Bible and other religious writing pointing toward an apocalypse and ''rapture," or ascent to heaven after 1,000 years.
Inevitable disappointments when the end of the world failed to materialize, he theorized, led to development of radical religious and survivalist cults such as the Branch Davidians of the Waco, Texas, conflagration.
Dr. Popkin also edited the 1999 ''Columbia History of Western Philosophy," among other works.
In 1962, he helped found the International Archives of the History of Ideas. Two years later, Dr. Popkin founded the Journal of the History of Philosophy.
A New York native, he also taught at the universities of Connecticut and Iowa, Washington University in St. Louis, Harvey Mudd College, and the University of California, San Diego, where he founded its philosophy department.