'When the facts change, I change my mind. What do you do, sir?" is a well known quote attributed to economist John Maynard Keynes.
It's meant to point up the absurdity of clinging on to beliefs after they have been proven factually wrong. We can hold as many contradictory opinions as we wish, but facts are sacred. We all know that knowledge is power and that education leads to enlightenment and clear thinking.
We know that an informed citizenry is preferable to an ignorant one and as believers in the democratic process we know that, as Thomas Jefferson wrote in 1789, "whenever the people are well informed, they can be trusted with their own government".
Which is why so many logically minded people are scratching their heads in frustration at what they see as others' refusal to acknowledge the inherent inaccuracies - okay, let's say downright lies - that are at the core of two current political campaigns, neither of which initially seemed to stand a chance of winning, but now seem worryingly likely to do so.
Last month, The Guardian newspaper printed an editorial on the UK European Union (EU) Leave campaign, subtitled: "Show some respect for the truth."
They, and others, have been assiduous in proving to us that many of the "facts" being cited by politicians such as Boris Johnson and Nigel Farage - from the amount of cash the UK gives to the EU, the cost of EU regulations to British business, the likelihood of immigration lessening if the UK leaves the EU, to the 'ban' on bendy bananas - were downright false and were easily proven to be so. But the only people who cared were those who knew and believed the truth already.
For those who are ideologically in favour of the UK leaving the EU, the truth of what their politicians were spouting mattered not a jot to them. Their gut feelings tell them that the EU is bad, therefore it is. Facts are irrelevant.
Similarly, over in the US, PolitiFact (a fact-checking organisation) found that just 9pc of all statements made recently by Republican presidential candidate Donald Trump are even remotely related to the truth.
In 2015, PolitiFact awarded him Lie of the Year when, out of 77 statements he made, 76 of them turned out to be "mostly false to false to pants-on-fire lies".
Yet as his statements are proved to be false, his popularity, like that of the Leave campaign, continues to soar.
As Mark Ritson wrote this week: "The debates have become childish slanging matches in which both sides throw bullshit in ever-increasing volumes at the other." He argues that the winner will be the side that can pull the emotional heartstrings the best, regardless of the facts.
It's not just in the political arena that 'whatever-belief-you're-having-yourself' trumps hard facts.
Give hardline anti-vaxxers, as they're called, irrefutable evidence that vaccinations have no serious ill effects and they will tell you that they are even more determined not to vaccinate their children. Give solid, scientific evidence that homeopathic 'medicines' are just sugar and water pills and those who swear by them will continue to believe that they work.
Prove to those who believe fluoride in our water causes all sorts of illnesses that there is absolutely no rational evidence to support their belief and they will just keep going believing that fluoride is the Devil's poison - again, regardless of the facts.
It's called the 'backfire effect'. Most of us like to assume that when our beliefs are challenged with facts, we will, like Keynes, change our beliefs.
However, researchers have discovered that in many cases the opposite is true; when a person's deepest convictions are challenged by contradictory thinking, their beliefs actually get stronger.
In today's world of 'all beliefs are equal', facts don't matter, 'feelings' do. Moral, cultural and scientific relativism is in, and validated, empirical 'truth' is out. Not only that, but there's never been a better time to get support and validation for not just factually incorrect but totally batshit crazy thinking. All you have to do is type "shape-shifting lizards run the world" into your laptop and you're guaranteed to get thousands of other fact-averse people supporting your claims and your gut feeling.
Or try it with "weapons of mass destruction in Iraq". Tony Blair did and he got the evidence he needed to invade the country. Evidence that Colin Powell later said was "a farrago of stovepiped intelligence, wishful thinking and utter bullshit".
The philosopher Harry G. Frankfurt first explained the difference between liars and bullshitters in his essay On Bullshit in 1986. Bullshitters - unlike liars, who are at least aware of the truth - don't care about the truth one way or another, it's irrelevant. Frankfurt's essay begins: "One of the most salient features of our culture is that there is so much bullshit." And he had yet to see the rise of the Internet and the way it has contributed to the spread of said bullshit.
Philosopher Stephen Law went further in 2011 with a book entitled Believing Bullshit: How not to get Sucked into an Intellectual Black Hole. In it, he succinctly explains how "Intellectual black holes are belief systems that draw people in and hold them captive so they become willing slaves of claptrap."
Law doesn't entirely agree with Frankfurt's description of bullshit, arguing that to some bullshitters, it matters that they believe the nonsense they spout to be true. Law explains strategies that people use for defending their own particular brand of claptrap. These include the "but it fits" strategy where "any theory, no matter how ludicrous, can be squared with the evidence, given enough ingenuity."
There's the "piling up the anecdotes" strategy ("but I know loads of people who used homeopathy and were cured") and my favourite one, "playing the mystery card", which Law explains is when bullshitters say: "Ah, but this is beyond the ability of science and reason to decide. You, Mr Clever Dick Scientist, are guilty of scientism, of assuming science can answer every question." This is often followed by that quote from Shakespeare's Hamlet: "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy." When you hear that, alarm bells should go off.
We now live in golden age of bullshit where academic rigour in any subject is seen as elitist and therefore undemocratic.
It's a case of "my uninformed opinion is as good as your evidence- backed research - and if you say it isn't I'll call you out as an intolerant bigot - or worse."
Or as Richard Hofstadter (author of Anti-Intellectualism in American Life) said: "Intellect is pitted against feeling on the ground that it is somehow inconsistent with warm emotion. It is pitted against character, because it is widely believed that intellect stands for mere cleverness, which transmutes into the sly or the diabolical." This may explain why so many American Republican voters preferred Trump over other, less fanciful candidates. Trump may not have the truth on his side, but he is most certainly emotive and he is a Class A bullshitter.
And so many of the great political and social issues today will be decided not by information from experts who have spent years studying in their fields, but by those who can shout loudest and longest; by people who "just know".
As Charles P Pierce put it: "Fact is that which enough people believe. Truth is determined by how fervently they believe it." But political ignorance is dangerous in a democracy. Research shows that the more threatened people feel, the less likely they are to listen to dissenting opinions, and the more easily controlled they are.
This plays right into the hands of demagogues such as Farage and Trump, whose whole raison d'être is to stoke up fear among their followers - "let's ban immigrants and build walls", etc.
But if people won't listen to the truth, how do we change hearts and minds?
One study suggested that people will actually update their beliefs if you hit them "between the eyes" with bluntly presented, objective facts that firmly contradict their preconceived ideas.
The problem, however, is that we've all become utterly terrified of offending other people's political, religious and cultural sensibilities, we're not in the habit of telling others that their beliefs are complete and utter hogwash.
We need to stop being so sensitive and call people like Trump and Farage out on their falsehoods, because the truth matters - and we better believe it.