Everyone enjoys a laugh at homeopathy supporters. It brings us all together. It's a way for smug, intellectual people to be all smug and intellectual. So there was barely-concealed glee this morning when an old tweet revealed Labour leadership contender Jeremy Corbyn believed in it, or rather that he did five years ago.

@leftoutside I believe that homeo-meds works for some ppl and that it compliments 'convential' meds. they both come from organic matter... — Jeremy Corbyn MP (@jeremycorbyn) March 5, 2010

Corbyn is considered a looney because he has actual political values which he refuses to compromise on, so this was opportunity to seal the deal. It's dispiriting to watch a man who espouses genuine convictions using normal language treated as some sort of museum piece, but that's where we are. One instinctively wants to jump to his aid, especially on something as minor and silly as homeopathy.

But belief in homeopathy serves a higher function than just screening out the gullible or providing a group hug for intellectuals. No matter how trivial it might seem, homeopathy provides a moral test because it functions as a litmus test for belief in objective truth.

There's no point going into the research – there's nothing left to say. Homeopathy is no more effective than a placebo. So when people say that it works, they are not just expressing a whacky view about alternative medicine, they are saying something more profound: that their intuition, or the experiences of someone they met once, overrule empirical evidence. It is a rebellion against reality.

One of the problems with smug intellectuals is that their attacks on homeopathy and other quackeries suggests the only enemies of objective truth are small groups of rich cranks. But the problem is far more deeply entrenched than that. It exists across the political classes, from secretaries of state to student activists.

Whatever you think of his politics, Michael Gove is plainly an intelligent man. But here he is yesterday speaking to the justice committee:

"The rights contained and articulated in the [European Convention of Human Rights] are admirable but we've got to make sure that they apply in a context where people believe that the culture of human rights is not being abused."

It's dressed up the standard managerial language, but this is the political equivalent of homeopathy. Notice the way that Gove does not raise any issues with the convention, but only with the perception of the convention. Perception is very often based on media output, which in this country leans heavily towards print due to rules on impartiality in broadcasting. So this subjective world view not only gives up on independent assessment of the facts, but also throws in its lot with the powerful newspaper editors who have the ability to shape public perception. It makes policy their plaything, a die cast to be applied to whatever mould they have managed to shape.

Gove's colleague at the Home Office, Theresa May, once decided to tackle health tourism. Except it didn't exist. She was told it cost just 0.01% of the NHS budget. Her response was to "refuse to quantify" the problem. It is a truly remarkable phrase.

Last April an NSPCC report found a tenth of 12-to-13-year-olds were "addicted" to pornography. We're completely unsure whether one can actually be addicted to pornography in a meaningful sense, but even if we were, the finding was based on an extremely dodgy survey by "creative market research" group OnePoll. Nevertheless it was widely reported and by the afternoon a government minister had promised that they would be taking action to address this imaginary problem.

Even the BBC is at it. Its political editor, Nick Robinson, argued that the corporation had been "too slow to detect and reflect public concern and anger" on immigration. Again we see the priority is not reality (academic research found the UK gained £20 billion from immigration between 2000 and 2011) but the perception of reality.

And that aversion to objective truth is eating away at the left as well, with a whole generation now convinced of the despairing nihilism of Michel Foucault's philosophy – basically that our only truths are personal ones dependent on our place in time and power structures. No discourse with young radicals lasts very long nowadays before a privilege is asserted, whether by race, gender, sexuality or gender identity. Usually this serves to close down the speaker's right to contribute, sometimes it is more nuanced and merely places their commentary lower down the scale of validity. Either way, there is here, once again, no such thing as objective truth – just the passing shades of our own subjective experiences, like shadows on a wall.

But here's the thing: objective truth does exist. Some things are right or wrong and that can be assessed objectively by how far they limit or maximise human freedom. Homeopathy does not cure cancer. Health tourism is not a major problem for the NHS. Immigration brings economic rewards to Britain.

Giving up on objective truth isn't just factually wrong. It is politically and morally wrong. It means we cannot hold the powerful to account, because there is no account upon which to hold them. It lets them off the hook.

In 1984 (sorry, but when you write blogs like this you have to quote Orwell, it's basically a contractual obligation), Winston Smith recognises that "freedom is the freedom to say that two plus two make four". His interrogator, O'Brien, asks:

"How many fingers am I holding up, Winston?" "Four." "And if the Party says that it is not four but five—then how many?"

O'Brien recognises that giving up on objective truth is a necessary precondition to tyranny. Once you forsake it, the authorities can get away with anything. That's why the first thing Christianity asks you to do is believe that three - the father, the son and the holy spirit - is one. Once you completely surrender your reason and an empirical assessment of the world around you, you belong to them. They've got you right where they want you.

That's why homeopathy, although it seems a small and eccentric idiocy, provides a useful function. It's not just a test of reason. It is a test of morality.