"Scientism” is the belief that all we need to solve the world’s problems is – you guessed it – science. People sometimes use the phrase “rational thinking”, but it amounts to the same thing. If only people would drop religion and all their other prejudices, we could use logic to fix everything.

Last week, US astrophysicist Neil deGrasse Tyson offered up the perfect example of scientism when he proposed the country of Rationalia, in which “all policy shall be based on the weight of evidence”.

Tyson is a very smart man, but this is not a smart idea. It is even, we might say, unreasonable and without sufficient evidence. Of course, imagining a society in which everyone behaves logically sounds appealing. But employing logic to consider the concept reveals that there could be no such thing.

There has always been a hope, especially as elites became less religious, that science would do more than simply provide a means for learning about the world around us. Science should also teach us how to live, pointing us towards the salvation that religion once promised. You can see this in any of the secular utopianisms of the 20th century, whether it’s the Third Reich, scientific Marxism, or the “modernisation thesis” of Western capitalism.

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First, experts usually don’t know nearly as much as they think they do. They often get it wrong, thanks to their inherently irrational brains that – through overconfidence, bubbles of like-minded thinkers, or just wanting to believe their vision of the world can be true – mislead us and misinterpret information.

Rationality is subjective. All humans experience such biases; the real problem is when we forget that scientists and experts are human too, and approach evidence and reasoned deliberation with the same prior commitments and unspoken assumptions as anyone else. Scientists: they’re just like us.

And second, science has no business telling people how to live. It’s striking how easily we forget the evil that following “science” can do. So many times throughout history, humans have thought they were behaving in logical and rational ways, only to realise that such acts have yielded morally heinous policies that were only enacted because reasonable people were swayed by “evidence”.

Phrenology – the determination of someone’s character through the shape and size of their cranium – was cutting-edge science. (Unsurprisingly, the upper class had great head ratios.) Eugenics was science, as was social Darwinism and the worst justifications of the Soviet and Nazi regimes.

Scientific racism was data-driven too, and incredibly well-respected. Scientists in the 19th century felt quite justified in claiming that “the weight of evidence” supported African slavery, white supremacy and the concerted effort to limit the reproduction of the “lesser” races.

It wasn’t so long ago that psychiatrists considered homosexuality unhealthy and abhorrent. There is at least one prominent, eminently rational psychiatrist who hasn’t come around on transgenders. And many scientists decided that women were biologically incapable of the same kind of rationality you find in men, a scientific sexism reborn in contemporary evolutionary psychology.

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In fact, creationism has a lot more in common with scientism than people such as Tyson or Richard Dawkins would ever admit. Like Tyson, creationists begin with certain prior commitments (“evolution cannot be true”, for example, substitutes for “science cannot be wrong”) and build an impressively consistent argument upon them. Just about everyone is guilty of some form of “motivated reasoning”: we begin with certain priors, and then find a way to get the evidence to do what we want.

The past mistakes of science should make us sceptical that it could be used to build a utopia. But, the scientists might say, science is most important for its ability to self-correct. Psychiatry has come around on homosexuality, for example. This may be true, yet it presents the precise reason why attempting to act only accounting for the “weight of evidence” is so flawed.