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Ever since the Enlightenment, rationality has been enshrined as a supreme value. We even describe our own species as the "rational animal". In his new book, "Irrationality: A History of the Dark Side of Reason", Justin E. H. Smith questions the accepted narrative that humans were shrouded in superstition and irrationality until the Greeks invented reason. Looking at philosophy and politics from antiquity to today, Smith - professor of the history and philosophy of science at the University of Paris 7–Denis Diderot - says that the evidence suggests that in fact, irrationality makes up the greater part of human life and history. Here, he discusses his arguments.

What brought you to this subject matter?

I wanted to write a book about animals, a sort of redux of Girolamo Rorario’s 16th-century treatise, which I discuss in the book, That Brute Animals Make Better Use of Reason Than Men. But I couldn’t get anyone interested in publishing such a thing. If you want to sell a book, it has to have plenty of what the journalists call "news hooks". So I began reading the news regularly, about Trump and the internet and whatever other chatter fills up our public space, and my conclusion was that, indeed, brute animals make better use of reason than men. And so the book is sort of the hybrid monster that comes from the fusion of these two strands: longstanding philosophical interests, plus an attempt to make sense of the present moment.

Your book challenges some accepted wisdom on humans and rationality. What are the most dominant assumptions about rationality and reason?

"Rationality" has a lot of implicit normativity packed into it, and the sense of calling something "rational" or "irrational" often slips away once we commit ourselves to eschewing normativity. "Rational" is, as Hartry Field nicely put it, often little more than an approval term.

How do you define rationality and irrationality?

They can’t really be defined in a succinct way that would cover all the cases in which they are meaningfully invoked. All one can do is survey the ways in which they are often deployed. "Rational" is typically used to describe people are behaving themselves, or systems that are functioning as they should. "Irrational" is thrown around when people are seen as being defiant of expectations or rules in some way or other. Sometimes this defiance is the result of a change in their cognitive state, eg., in drunkenness, stonedness, dreams, orgiastic revelry, delight in storytelling, and in such myth-spinning and melting of the ego into the crowd as happens in raves and rock concerts and Trump rallies. Sometimes it is just a matter of defiance or rejection.

Why are humans so irrational?

Because they are all going to die. That makes adherence to the "right" path in life, the one that rational-choice theory would advise, an ultimately futile project. So being rational is irrational. But being irrational is irrational too, somewhat more self-evidently. And so you can’t win.

Are there any benefits to being irrational?

Plenty. Typically the highs are higher and the lows are lower. But the problem is that insofar as we are calculating the expected "benefit" of an irrational course of action, we’re still thinking rationally. So it’s basically incoherent to advise people to be irrational. Better to just say: go ahead and be irrational if you feel like it; it’s not my problem.

Many commentators have suggested that we're living in a uniquely irrational/"post-truth" moment. What do you think?

Well, our brains are still the same, but something changed in the history of telecommunications technology over the past decade or so that has made possible an amplification and a mainstreaming of the sort of irrational effervescence that for a long time was kept on the margins. I mean, within just a few years of the rise of social media, a professional troll was elected president of the most powerful country in the world. That’s something the left didn’t come close to achieving during the peak of its own effervescence in the late 1960s. It’s as if the right-wing populist equivalent of a Wavy Gravy-figure has been thrust into highest office. What has made the difference in terms of power to warp the way people think and speak and vote, is social media.

How does it compare to historic examples of irrationality?

There are important respects in which the present moment sounds an echo of the chaos in the 16th and 17th centuries following the rise of print culture, and what the book historian Ann Blair has called "early modern information overload". That period saw a violent recalibration of norms and rules to accommodate the new realities of the information landscape and their political consequences. But a lot of people were brutally suppressed by the Inquisition before these new norms took hold, a lot of people were burned at the stake for translating the Bible into the various national vulgates, etc. The rise of fascist leaders propelled into power by WhatsApp, the ethnic cleansing and genocides stoked by Facebook, the insatiable appetite of the roving "cancel culture" mobs on Twitter: this could all perhaps have been predicted by anyone who had well studied the upheavals of the Reformation and their link to the spread of print technology.

Can we ever eliminate irrationality? Would that be desirable?

No, and no. It’s more like tooth decay than, say, polio. Once you become aware of the threat it poses, you do not knock out your teeth. You just try to brush them.