Isaac Asimov was an American author and professor of biochemistry at Boston University, best known for his works of science fiction and for his popular science books. Asimov was one of the most prolific writers of all time, having written or edited more than 500 books and an estimated 90,000 letters and postcards. His works have been published in nine of the ten major categories of the Dewey Decimal System. Asimov is widely considered a master of hard science fiction and, along with Robert A. Heinlein and Arthur C. Clarke, was considered one of the "Big Three" science fiction writers during his lifetime. Asimov's most famous work is the Foundation Series; his other major series are the Galactic Empire series and the Robot series. The Galactic Empire novels are set in earlier history of the same fictional universe as the Foundation series. Later, with Foundation and Earth (1986), he linked this distant future to the Robot stories, creating a unified "future history" for his stories much like those pioneered by Robert A. Heinlein and previously produced by Cordwainer Smith and Poul Anderson. Asimov wrote the Lucky Starr series of juvenile science-fiction novels using the pen name Paul French. Asimov also wrote mysteries and fantasy, as well as much nonfiction. Most of his popular science books explain scientific concepts in a historical way, going as far back as possible to a time when the science in question was at its simplest stage. Examples include Guide to Science, the three-volume set Understanding Physics, and Asimov's Chronology of Science and Discovery.