Asked whether great books are still being written, Harvey Mansfield - who has spent 61 years teaching political philosophy at Harvard - offered a stark answer: for the entire 20th century, he could name only two authors who reached that level. "On the level of the canon of great books, I guess I would say in the 20th century, Heidegger. I would add Strauss to Heidegger, though I know that that's a controversial statement."

The admission, made during an extensive Conversations with Tyler interview, "Hvylya" reports, prompted interviewer Tyler Cowen to press the point. In the 18th and 19th centuries, one could easily name a dozen great books per century. The 20th century, with its larger population and higher literacy, produced two authors. "What changed?" Cowen asked.

Mansfield's diagnosis pointed to a philosophical shift that began in the early 19th century. "Philosophy has been historicized such that people doubt that a great book is possible because it's not easy or possible for a thinker to think outside his time," he said. The belief that every thinker is trapped within historical context has eroded the ambition that once drove philosophers to write for eternity.

A great book, in Mansfield's definition, "is always one that is written in a time, but for the sake of the future, and the possibilities of what will happen, and for other times." He invoked Thucydides, who described his own work as "a possession for all times." That ambition - to produce something that transcends the era of its creation - "has left us," Mansfield said.

He was careful, however, not to overstate the crisis. "I'm not sure that there has been a change. I'm not sure that that is a useful speculation. It's better to not expect a great book," he said. Great books have always been rare. What has changed is not the frequency but the aspiration - modern authors are no longer trying to write books that outlast their century.