It’s a heady time in politics, when so much of our debate centres on what it means to lie. If Boris Johnson didn’t think he was at a party, yet others at the party did, is that a lie? If Rishi Sunak comes out studs first on the matter of his wife’s non-domiciled status, claiming it as a mere function of her nationality, only for that to be debunked by tax experts in a matter of minutes, was that a lie? Or merely a man experimenting with different lines, to see which would hold? These are obviously the pressing questions of the moment, but I find myself more transfixed by a wonderful book, Tremors in the Blood, published today by the tech journalist Amit Katwala. It tells the story of the lie detector, from the first, gripping murder case for which it was conceived, up to its use today in the justice system. Currently, we exist in a bizarre contradiction: scientifically, lie detection has been debunked. It simply isn’t possible to override a person’s words and listen to their body instead. The author quotes two academics from Northumbria Law School, Marion Oswald and Kyriakos Kotsoglou, who concluded after extensive meta-analysis: “We cannot stress enough that ever since the first deployment of the polygraph, appellate courts, scientific organisations, and last but not least academic discourse have continuously and nearly unanimously criticised, and rejected this method as unscientific.” That is not what’s puzzling, either to Katwala or to any casual observer; rather, if the polygraph has been so roundly discredited for so long, why do probation services, as well as the justice system in general, still use it? There have been periods when parallel ideas – which sought, with the certainty and neutrality of science, to clean up the messiness of human veracity and behaviour – have been tested and discarded. In the 60s and 70s, psychiatrists and forensic psychologists were very excited by the idea of electric aversion therapy for sex offenders – literally, showing paedophilic images to offenders at the same time as administering electric shocks. It worked on paper; it did not work on humans. The motive driving polygraph enthusiasts was similar – wouldn’t it be great if science could simply solve a problem, render it in black and white? But the dream was different: lie detection seeks not to modify a behaviour but to reach beneath questions of judgment and uncertainty, and discover the truth as something immovable. In many ways, the idea of the polygraph contains the same fallacies as torture, without the cruelty: that the mind can lie, but the body can’t. In fact, the body can lie perfectly well. To crave a world in which deceit is impossible is a natural thing. You might lose a bit of poetry and drama, but you’d gain a lot of clarity. It’s a piquant time to consider that craving, since the conventions of our politics are built on that perfect world. We conduct our meaningful debate in an environment in which it is considered impossible for a person to be lying. This is not naivety, but a necessary premise: if parliamentarians may or may not be telling the truth, if the veracity of their statements has to be tested against the facts, then that’s not a debating chamber but a rather ornate court of law; you cannot make law in a courtroom. So even if for centuries there have been members whose truthfulness you doubt a bit, those bad apples have never been reason enough to overturn the presumption of honesty, not because the apples weren’t bad, but because the presumption is so important. One felt for Keir Starmer on Tuesday. It looked like an open goal, but there are only so many times you can call the prime minister a liar before it dents your own professionalism and standing. You don’t sound like a politician, you just sound like one of his wives. But even if it can feel ridiculous, to consider parliament as a utopia of truth, this crisis has only underlined how important it is; long after this government has resigned or had their resignation handed to them by the electorate, we will remember the impossibilism of civic life where honesty couldn’t be assumed. Zoe Williams is a Guardian columnist