“These are things that don’t necessarily need to be true. As long as they are believed.” This was what Alexander Nix, the chief executive of Cambridge Analytica, told an undercover Channel 4 News journalist as part of an exposé into the murky world of data harvesting and alleged manipulation of elections. It could turn into the defining comment of our age – an epitaph for truth.

Facts, backed up by experts and statistics, have to queue up behind prejudice, conspiracy theory and distortion. In our new post-truth world, things that are believed matter more than facts. And this does not only apply in the controversy over Cambridge Analytica and Facebook, but in Jeremy Corbyn’s response to the poisoning of Sergei and Yulia Skripal.

Here are the facts. The former Russian spy, who was described by the Kremlin as a traitor, and his daughter lie stricken in hospital more than a fortnight after being taken ill in Salisbury. Scientists at Porton Down, the chemical and biological weapons laboratory, have examined the substance and concluded that it is from the group of nerve agents known as Novichok, developed by Russia.

Russia election 2018: in pictures Show all 27 1 /27 Russia election 2018: in pictures Russia election 2018: in pictures People attend a rally in Manezhnaya Square near the Kremlin during the ongoing presidential elections. EPA Russia election 2018: in pictures The members of the local election commission open a ballot box for counting at a polling station during the presidential elections in St. Petersburg. EPA Russia election 2018: in pictures Russian opposition leader Alexei Navalny and Presidential candidate Ksenia Sobchak attend a debate at the "Navalny Live" YouTube show in Moscow. Reuters Russia election 2018: in pictures Members of a local election commission count votes during Russia's presidential election in the small town of Krasnyi. AFP/Getty Russia election 2018: in pictures President Vladimir Putin walks out of a voting booth at a polling station during Russia's presidential election in Moscow. AFP/Getty Russia election 2018: in pictures An elderly woman casts her ballot at her house during Russia's presidential election in the village of Khrapovo. AFP/Getty Russia election 2018: in pictures Presidential candidate Ksenia Sobchak votes inside a polling booth in Moscow. AP Russia election 2018: in pictures Russian Communist Party presidential candidate Pavel Grudinin votes at a polling station in the Sovkhoz Imeni Lenina, outside Moscow. AFP/Getty Russia election 2018: in pictures A woman with her dog lines up with Russian military personnel to vote in the presidential election in Moscow. AP Russia election 2018: in pictures A man casts his ballot at a polling station during Russia's presidential election in the small town of Krasnyi. AFP/Getty Russia election 2018: in pictures Presidential candidate Vladimir Zhirinovsky casts a ballot at a polling station in Moscow. Reuters Russia election 2018: in pictures An elderly woman fills her ballot at her house as a member of a mobile Russian election committee visits residents of the village of Sovyaki. EPA Russia election 2018: in pictures A child plays at a polling station during presidential elections in St.Petersburg. AP Russia election 2018: in pictures A man casts his ballot at a polling station inside Kazansky railway terminal. AFP/Getty Russia election 2018: in pictures Vladimir Putin casts his ballot. Reuters Russia election 2018: in pictures A man casts a ballot, during the presidential election, inside the Russian Embassy in London. Reuters Russia election 2018: in pictures People leave a polling station during the presidential election in Moscow. Reuters Russia election 2018: in pictures Presidential candidate Sergei Baburin, leader of the nationalist People's Union party, votes at a polling station in Moscow. AFP/Getty Russia election 2018: in pictures Voters look at a poster displaying presidential candidates at a polling station in the ZIL cultural centre in Moscow. AFP/Getty Images Russia election 2018: in pictures An Orthodox Jewish Russian citizen casts his ballot at a polling station for the Russian presidential elections in the Sergei Building at the Russian compound in Jerusalem. EPA Russia election 2018: in pictures Players of the Russian national soccer team, including Vladimir Gabulov and Yuri Zhirkov, visit a polling station during the presidential election at the Novogorsk training centre outside Moscow. Reuters Russia election 2018: in pictures President Vladimir Putin shakes the hand of a polling station staff member during voting. AFP/Getty Images Russia election 2018: in pictures A man votes at a polling station in Moscow. Rex Russia election 2018: in pictures Presidential candidate Ksenia Sobchak casts her ballot by scanning it in at a polling station in Moscow. EPA Russia election 2018: in pictures Policemen guarding the General Consulate of Russia in the Ukrainian city of Kharkiv watch an Ukrainian activist touching the nose of a dummy embodying Russia's president in a coffin. AFP/Getty Russia election 2018: in pictures A woman with a dog reads her ballot at a polling station in Moscow. AFP/Getty Russia election 2018: in pictures A man walks out of a voting booth at a polling station in the village of Novye Bateki. AFP/Getty

Theresa May gave the Russian government 36 hours to explain whether the Salisbury attack had been carried out at the behest of Moscow or that it had lost control of its own chemical weapons – and either way Russia would be in breach of the Chemical Weapons Convention on the declaration of such agents. Vladimir Putin’s government refused to respond to the Prime Minister’s ultimatum. Meanwhile, Britain has the support of the US, Germany, France and other countries in saying Russia was responsible.

The Labour leader’s response has not been so forthright. Last week, he urged the British Government not to rush to judgement on Salisbury. His most recent comment, on Radio 4’s World At One on Tuesday, was this: “I asked the Russians be given a sample so that they can say categorically one way or the other” and he urged the British Government not to “shoot from the hip”.

He also said: “Would I do business with Putin? Sure.”

Corbyn, and his communications director Seumas Milne, ask for Salisbury to be put into context – specifically, the context of how Western intelligence got it wrong on Iraq and weapons of mass destruction, and how rushing to judgement in 2003 had been a mistake. Indeed, they were right about Iraq – in the sense that there was a rush to judgement, and that there were no WMDs, although it was the way the politicians reported the intelligence, not the intelligence itself, that was at fault.

John McDonnell: 'All evidence' points to Putin being responsible for Russian spy poisoning

It is cynical of Corbyn and his aides to use this week’s 15th anniversary of the invasion of Iraq to support their equivocation over Salisbury. The comparison between Iraq and Salisbury is a feeble one: WMDs never materialised in Iraq; in Salisbury they are the reason why two people are fighting for their lives, a third is recovering in hospital, and hundreds of others were urged to follow decontamination procedures.

While the UK and US failed to get support in the UN for military action against Saddam Hussein, an international consensus is building against Russia over Salisbury.

Yet just as Corbyn asks for intelligence on Salisbury to be taken into context, he cannot expect his own position on Salisbury to be seen in isolation. When the Labour leader casts doubt on May’s response or Russia’s involvement, he does not do so as an objective commentator on geopolitics: he has a long record of opposing Nato and the West, and of supporting Russia – as does Milne. When Corbyn asks that Moscow be given a sample of the nerve agent “so that they can say categorically one way or the other”, he makes clear that the arbiters of who authorised the first use of chemical weapons on the streets of Britain are not scientists at Porton Down, nor the British intelligence services, nor the British Government – but the Kremlin, and a president with a track record of distortion, manipulation and retaliation against his enemies. Corbyn’s own opinion and background, rather than the cool-headed objective facts, set the scene for his position on Salisbury.