Hold the front page: Emma Thompson, protester against climate change, scourge of the fossil fuel industry, herself takes aeroplanes. There she was, bold as brass, in a seat, about to burn through rainforests of unknown dimensions with her cross-Atlantic travel, on the cover of the Mail on Sunday.

On some level we must have known this all along, since mostly she is in the UK, sometimes she is in the United States, and we know (although sometimes I wonder) she isn’t magic. But still, doesn’t that put the old tin lid on it? What’s the point of it all, when even people who care about the future and believe in science aren’t perfect?

You might eschew planes but get caught eating a burger; you might be a vegan, but what's the environmental cost of soya?

The counsel of perfection, looking for the hypocrisy in anyone taking a position distinguishable from naked self-interest, is actually slightly worse than a counsel of despair: it takes pointlessness one step further, besmirching everyone, sucking the energy out of everything. Nobody will ever be good enough on these terms. You might eschew planes but get caught eating a burger; you might be a vegan, but have you seen the environmental cost of soya?

The climate emergency, being a crisis, needs radical action; transfer that on to any individual, and the only way not to be a hypocrite is to live off-grid, the downside of which is that, now, you have removed yourself from culture, and even if anyone gets to hear of you it’s as a crank.

This, by the way, is not a new technique for resisting change: Martin Luther got a hell of a shellacking for having an affair with a nun while trying to take down Catholicism. I won’t pretend he stayed true to the spirit, or indeed the letter of monastic law. But the movement was bigger than the man and, in the end, the wisdom of crowds deemed a corrupt institution, spanning and dominating millions of lives, more important a foe than an insightful, horny guy with ego oversupply.

If “let he who is without sin cast the first stone” is a timeless rhetorical device, its sheer prevalence in modern discourse has coated every issue with ad hominem gotcha and irredeemable shame. But that stone, recall, was to kill an adulteress. So if, by some extraordinary happenstance, there was nobody without sin, and no adulteresses ended up dead, Jesus (I think) would have been fine with that.

This moral code was never designed to kill collective action in its crib, to snuff out human altruism because its proponents weren’t themselves quite altruistic enough; it’s there, at the risk of oversimplifying, to stop us doing vile things to one another. To use it any other way is to turn morality on its head, so that it becomes not an expression of the best of us, but a creed of “we’re all as bad as each other”.

By these standards, the left has no case to make until it can be voiced through a leftwing messiah – which is ironic, because we’re not the ones who believe in messiahs. We believe in reason; which is now another, even more consequential, sticking point.

It has become voguish on the right to reject science, sometimes in favour of a godhead, other times for a strong man who merely says it ain’t so. While climate-change denial is gaining in extremism, it is actually the anti-vaccination lobby that is the more chilling (there is significant overlap, of course): not because children needlessly and demonstrably die as a result of their fantasies, but because vaccination has been a matter of settled science for years. Not even the BBC would try to create “balance” by putting a critic of herd immunity against a proponent.

So when a movement gathers steam against vaccination, the message is: there is no place for authority here, and there is no appeal to evidence or observation that will be decisive. This matter is infinitely contestable, purely political. And where the final arbitration of science or reason will not be admitted, politics itself becomes an act of violence: with no right or wrong, there’s only shouting and shouting louder.

Traditionally, the “reasonable” world has accommodated a fair amount of anti-morality and plenty of anti-science for the sake of compromise and inclusivity. There hasn’t been an American election in which both sides agree on evolution, for example, since the 1920s. Yet arguably you could trace a direct line from the acceptance of biblical supremacy in public life to the acceptance of a birther in the White House; once the principle of objective truth had been surrendered, all it took was audacity for the territory of reason to be swarmed by malicious superstition.

The post-Enlightenment settlement claims to prize an evidence base above all, but it also – having assessed all the evidence – finds social democracy best served by seeing things from all possible perspectives. This principle of pluralism finds it easier to accept a broad political movement that believes the world was created in seven days than to insist that the minimum price of entry to the public discourse is, if not to know much about geology, at least to believe geologists. But we’re now in a world of anti-vaxxers and climate-change deniers, Holocaust deniers, fake-news peddlers, public “intellectuals” who believe in racial supremacy, European governing parties that would see women returned to the hearth, and news sources polluting every creative and humanist act with the hint that it came from a place of hypocrisy, attention-seeking and self-interest.

Some of these things look very naturally aligned, and some miles away from one another, and yet they all share the same wellspring: a knowing readiness to set the world on its head rather than its feet, to reject the possibility of evidence and objectivity, still less the hope of any unanimously agreed moral code.

Nobody will ever be perfect enough to fight any progressive cause, on the basis that everyone, being fundamentally ignoble, most probably has ignoble aims. No case, however self-evident, can fight its own corner once the authority of evidence is in dispute. No lesson can be learned when science and history are contested by bare assertion, and fiction becomes just a funner version of fact.

Liberals, paradoxically, need to become much more absolute about what they will and will not accept, right down to – though it may sound unlikely – what can and cannot be levelled against someone like Emma Thompson.

• Zoe Williams is a Guardian columnist