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It鈥檚 a story we can鈥檛 stop telling ourselves. Once, humans were benighted by superstition and irrationality, but then the Greeks invented reason. Later, the Enlightenment enshrined rationality as the supreme value. Discovering that reason is the defining feature of our species, we named ourselves the 鈥渞ational animal.鈥� But is this flattering story itself rational? In this sweeping account of irrationality from antiquity to today鈥攆rom the fifth-century BC murder of Hippasus for revealing the existence of irrational numbers to the rise of Twitter mobs and the election of Donald Trump鈥擩ustin Smith says the evidence suggests the opposite. From sex and music to religion and war, irrationality makes up the greater part of human life and history.

What led you to write a book about irrationality?

JS: I had long supposed that human thought and behavior have been a relatively static thing for the past 200,000 years, that there is a fairly narrow range of species-specific responses to the world around us, and that these are not going to fundamentally change until or unless we become a different sort of animal. The past few years have tested this long-held assumption. I came to feel that the world was going mad, that many people, including many I know and love, were now speaking and reasoning as if they had passed through to the other side of a looking-glass, or had come back from the other side, and were now communicating in a frenetic glossolalia or in pretend robot-voices. And it terrified me. I began to wonder whether this is not a normal process of disillusionment one can expect to go through at a certain stage of life, when the scales fall from our eyes and we realize that human beings have been bonkers all along and that society is just a flimsy tarp that camouflages this madness, or whether, instead, there really is something important about the present moment that is bringing the irrationality out, like methane from below the ice of the melting tundra. It seemed to me the best way to answer this question would be to investigate it historically, with a maximally sweeping view, attempting so to speak a genealogy of irrationality, one which reaches back into the past, but always with an eye to understanding the present. 

You say that irrationality is ineliminable from human life. Why do you think humans are so inherently irrational? Why doesn鈥檛 it seem to matter to us if there are facts that directly contradict our irrationally-held beliefs?

JS: The point of emphasizing its ineliminability is that, again at least until we become a different sort of animal by evolution or by genetic engineering, harm to human beings will be reduced if we understand irrationality as something to be managed, in the way we manage our proneness to tooth decay and do not simply knock our teeth out and replace them with dentures, rather than as something to be obliterated, like polio or cancer. Historically, every attempt at structuring society in a perfectly rational way has been a folly, and has resulted in tremendous individual suffering, in part because the human beings made to endure such political projects remain exactly the same inwardly as humans in those societies that have found effective ways to manage all our dark impulses and unjustifiable but beautiful attachments rather than simply to suppress them. In this respect my argument is kind of boring in its centrism and its attachment to the golden mean, but it also discerns, I think, an important dialectical connection between the two poles in question: do not become too devoted to reason, or you will be pulled over into the opposite extreme. 

Can groups like LessWrong ever really eliminate irrational decision-making as it relates to artificial intelligence and business operations?

JS: Of course not. As I say in the book, they鈥檇 be a lot better off just reading some Virgil or Shakespeare and not worrying so much about whether it鈥檚 helping them to better apply Bayesianism to their daily lives, rather than acting as if human flourishing is equivalent to making rationally justifiable choices. I mean, obviously, if you spend your days writing Harry Potter fan-fiction, which seems to be a thing in that subculture, something has gone very wrong, and no amount of formal epistemology or probability theory can rescue you from what appears to an outsider to that subculture as an obviously bad choice, not just of how to spend one鈥檚 time, but of a whole form of life. 

Is there any benefit to thinking or behaving irrationally?

JS: Sometimes, but it can also kill you, so you need to make your decisions wisely. Sometimes it鈥檚 a good idea to smoke; sometimes it鈥檚 a good idea to be foolhardy in combat; sometimes it鈥檚 a good idea to free solo climb El Capitan. Other times it鈥檚 not. The big mistake is to suppose that one can turn to philosophy to find 鈥榬ules for living鈥� that would dictate generic principles applicable in all circumstances, rather than acknowledging that the only answer is, often, what may be called a radical choice, ungrounded in any principle or rule. 

So, what can we take away from all of this?

JS: The book is an essay and not a theoretical or argumentative work, which means that it is a contribution to a genre in ill repute among academics, and runs the risk of being dismissed by my philosopher-peers as conveying little more than what is called on Twitter a 鈥榤ood鈥�. I can live with that. Moods can be diagnostically very useful, and we certainly know more about what people were really thinking, say, in the 16th century, when we read Montaigne than when we read Francisco Su谩rez. In any case if the book were making a theoretical argument, it would almost certainly be wrong. In being honest about its true character as an essay, I may hope that at least some readers will be able to share the mood of it, and perhaps thereby to accompany me in the project of becoming a bit wiser. 

Justin E. H. Smith is professor of the history and philosophy of science at the University of Paris 7鈥揇enis Diderot. His books include The Philosopher: A History in Six Types (Princeton). An editor at large of Cabinet Magazine, he also writes frequently for the New York Times, Harper鈥檚 Magazine, and other publications.