Very few humans actively strive to do the wrong thing, or to have false beliefs. Most of us do what we sincerely think is right, and believe what we sincerely think the evidence points to. The trouble is, what we think is right and what we think the evidence points to is often different from what others think is right or what they think the evidence points to. We are always enlightened “by our own lights”.

So when moralists say that people doing bad things must be acting out of “self-interest”, say, or when scientists say that people who have mistaken views should adopt “evidence-based” theory instead, they reveal that they are in the grip of a strange sort of parochialism. They seem not to acknowledge the mere existence of alternative opinions to their own. It isn’t simply that they think their own opinions are right and that alternative opinions are mistaken – we all do that – rather, they assume that there simply are no such alternative opinions. Hence those who act in ways they disapprove of are acting out of “self-interest” – self-interest conveniently being an entirely non-moral motive rather than a more troubling one inspired by a different moral theory from their own. Or again, those who believe non-approved opinions aren’t simply interpreting the evidence in a different way or counting different facts as evidence – they are ignoring evidence altogether, according to this way of thinking. Thus people who disagree with the approved opinion are not simply opponents, but “deniers” who must be scoundrels (or may be suffering from a debilitating mental illness).

This sort of parochialism would be a venial sin if those who committed it could correct themselves and move on. But that cannot happen – this is an unusually immovable vice, because it has its own incorrigibility built into it. In effect, it is designed not to allow correction. Why? – We engage with people who disagree with us, whose opinions are opposed to our own, but none of us could be bothered to engage with people who simply don’t have an opinion at all. Anything they say can be waved away as an irrelevance, because they have changed the subject. If what masquerades as an alternative opinion has nothing to do with evidence, then it isn’t an opinion at all, just a rude noise made by an insincere scoundrel.

For example, in disputes over moral questions, Kantians and utilitarians speak different languages because they appeal to very different basic principles. Too often, one side assumes the other has so misconceived morality that it “doesn’t have a moral view at all”, so that all its verbiage is an expression of something else. If so, anything said in its defence is irrelevant and can safely be ignored. During my own brief career as an academic, I saw far too much of this very unbecoming and lazy habit of thought. It is a discredit to anyone who professes to be a thinker.

This sort of dispute doesn’t just engage philosophers. Both sides to practically every political dispute – from the Falklands to Northern Ireland to Israel – are entirely sincere, but talk right past one another because they assume the other side is not sincere.

In discussing science, Baconian inductivists and hypothetico-deductivists speak different languages, because they count different sorts of facts as evidence, and different practices as scientific. It isn’t that one side is “evidence-based” and the other isn’t, but rather what counts as evidence to one side doesn’t count as evidence at all to the other. (For example, Newton rejected hypotheses, or so he thought, and Popper rejected induction, or so he thought.)

Thomas Kuhn saw all this decades ago, not in the philosophical discussion of science but in science itself – proponents of an old “paradigm” die out rather than convert to a new paradigm. They cannot convert, because the new way of thinking is so alien to them that the meaning of the terms they use – and even what they count as evidence – is different.

Personally, I find it very odd that anyone who has understood Kuhn’s The Structure of Scientific Revolutions could use the word ‘denier’ or ‘denialist’. I want to ask: Have you learned nothing from the mistakes of others?