But what’s the point of curating ideas with such a capacity? Why go to an appalling lecture and why buy tickets to a festival offering a variety of different ways to feel revolted? Longstaff explained: “Our objective in presenting dangerous ideas is not that these ideas be promoted or adopted, but simply that they be encountered and, thus, assessed on their merits. … We believe that ideas of all kinds are best exposed to the light of reason and discernment.”

In other words: The ideas will exist without the festival, so we might as well have an orderly process for looking into them.

Still, one man’s dangerous idea is another man’s conventional orthodoxy, as Longstaff acknowledged. “We knew that, for some people, some of the ideas would be innocuous—there is, after all, no danger in ideas with which you agree,” he wrote. The talk that kicked off the first festival five years ago was a case in point. Christopher Hitchens’s keynote address, “Religion Poisons Everything,” was provocatively packaged, but the content likely didn’t shock most of the young urban liberals who, according to Mossop, buy many of the tickets to the festival in the first place. Hitchens’s 2009 FODI talk took place five years after the publication of Sam Harris’s The End of Faith, one of the founding works of the so-called New Atheist movement that Hitchens came to represent. The book was unthreatening enough to spend 33 weeks on the New York Times paperback best-seller list.

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This year’s program featured Kajsa Ekis Ekman, a Swedish activist and journalist with the dangerous idea that, as her Saturday talk was titled, “Surrogacy is Child Trafficking.” She has her own perspective on what makes an idea dangerous, for good or ill. “Fascism is, of course, a dangerous idea,” she tells me. “And I think not all dangerous ideas are good or should be let loose in that sense. … But then again I would say that there are also ideas that are dangerous in the good way, because they stimulate you to question everything that you’ve taken for granted, and especially ideas that question the existing economic and social order.”

Australians have had occasion to question the practice of child surrogacy in recent weeks as the story of “Baby Gammy” has unfolded in the national and international media. A 21-year-old surrogate in Thailand gave birth to Gammy and his twin sister Pipah on behalf of an Australian couple early this year. But when Gammy was born with Down syndrome, his parents reportedly took his healthy twin sister to Australia and left Gammy behind with the surrogate. The case has brought new attention to the trade of paid surrogacy, which is illegal in many countries, including Australia. But couples from all over the world hire surrogates in countries where doing so is legal or loosely regulated, as it is in the U.S., India, Thailand, Mexico, and Ukraine. “All of a sudden, this is a huge issue here,” Mossop says.