US liberals and leftists who won’t vote for Clinton, even though that decision will help Trump, who is even worse, are an interesting case study. It reveals a deep difference, not between liberals and conservatives, but between “principlists” and “consequentialists”.

Consequentialists do what they have to do to get the best available outcome, even if the means – and the end – fall short of their ideal.

Principlists feel an inherent wrongness in doing anything against principle, even if the result is an outcome even further from their ideal.

They’re two totally opposed mindsets – ways of thinking about how to choose action – with little scope for persuasion between them.

I think this is the root of the left’s tendency toward schism. Do you compromise to get power and make some improvements? Or cling to an ideal vision and fight for it at all costs? (The cost being perpetual rule by your opponents and no progress at all.)

Most present and historical splits on the left boil down to this: Social Democrats v Communists, Stalin v Trotsky, MacDonald v Labour, Blairites v Corbynistas. Other ideologies and parties are affected too: UKIPpers splitting the Tory vote. Clegg et al in Coalition, against Liberal Democrat members and activists.

Perhaps it was even the cause of religious schism too, when religion, not politics, was people’s primary moral identifier and motivation. For example, should the Byzantine church have set aside minor differences and unify with Rome, to get military help against the Muslim Ottomans? In that case, the principlists won: unification was stalled, no help came and Byzantium fell. Eastern Christianity stayed pure – and was all but wiped out.

Is this just the luxury of hindsight? I suspect even if you could have convinced the orthodox Byzantines what would happen, they’d still veto rejoining Rome.

You will never persuade someone who thinks that value is in the action to commit a compromised action for a better result. Likewise, you will never be understood by someone who thinks that value is in the result, if you throw the result for a principled action.

The principlist/consequentialist divide is also why it’s so hard to make progress on social issues like prison reform, drugs and sex work.

Ask someone why we punish criminals. If they say deterrence, prevention or rehabilitation, they’re a consequentialist thinker. If they say something along the lines of, “because they deserve it,” they’re a principlist. No initiative, however much it improves outcomes for deterrence etc, will be acceptable if it isn’t sufficiently punitive in and of itself.

Suppose we ran a pilot scheme where instead of jailing burglars, we gave them £10k and put them on a job scheme. Suppose the results of the trial conclusively proved it significantly reduced re-offending, and was cost effective (cheaper than prison).

This should be a no-brainer. Rehabilitating burglars, making society safer, for less cost. But you know people would object. Principlists would be furious: burglars deserve to be locked up, not rewarded with cash and jobs.

It doesn’t matter that the principlists’ plan – lock them up – results in more crime. The response itself, not the outcome, is what matters.

It’s important for progressives to understand this. It’s the basis of some of their coalitions, but also a great hurdle for their causes.

Pro-abortion people can persuade anti-abortion consequentialists that banning drives it underground: not fewer abortions, just more harm. But neither group will get that through to anti-abortion principlists. For them, abortion is just wrong, and should never be facilitated. That remains true, even if banning abortions demonstrably results in more abortions.

Similarly, we know that prescribing heroin to heroin addicts saves lives, reduces crime and gets them off heroin. But there’ll always be people who think it’s moral insanity. An addict is an addict: you don’t reward them with the object of their addiction. No matter that it’s provably the best way to stop them being an addict, and therefore a win-win for society.

You live in an area blighted by drugs? The NHS giving drugs to addicts would literally prevent you being mugged and burgled several times over. Your life would be better. Your neighbourhood would be saved. The addict recovers. So how about it?

<Principlist shakes head> “Uh-uh.”

You sure? Fighting the “war on drugs” will ensure your neighbourhood continues to be a warzone, controlled by criminal gangs.

“The police should crack down on the gangs.”

They are; it makes it worse. Removing gang leaders results in even more violent struggles between others trying to replace them.

“They should go after them too.”

This will literally continue until everyone here is dead. That’s your solution?

Of course, the principlist doesn’t think such issues through to their inevitable consequences. And that’s the point: consequences simply have no place or relevance in their moral judgments.

This brick wall – the utterly different ways of placing value on action – comes up again in discussion of immigration.

You know the argument: “We’re full. We don’t want immigrants coming here, taking our jobs and using our public services.”

There are many pre-rational reasons people believe this: tribalism, media bias, impoverishment by free-market globalisation. But another foundation for it, which has been under-analysed, is the pure principlism of it.

The factual argument is easy to counter: immigrants are a net contributor to the economy. More immigrants, more demand, more jobs, more tax returns, better public services. Fine if you’re arguing with a consequentialist. But why does it repeatedly fail to convince others?

Because it’s the principle: immigrants haven’t paid for the NHS, so they don’t deserve it. No matter that by being here, they’ll improve it. That’s giving them something they don’t deserve – worse than everyone getting more!

Obviously you can tell by the tone of my argument that I’m a consequentialist and think principlism is stupid. But my point is that it’s not actually stupid. It’s not a difference in intelligence or rationality, but in what you assign value to.

And even if you’re reading this thinking, “I’m a good, rational non-idiot who understands consequences and that’s why I’m progressive”… You probably have principlist views on some things too.

What if, for example, you could torture someone into revealing a secret government torture programme – thereby putting a stop to it? Are you pro-torture now, if it reduces torture? Or absolutely against torture in any circumstance?

Principlism and consequentialism are two mindsets which we all share to some degree. Some tend towards one, others to the other. And there’s no way to rationally argue from one to the other, either.

If we have to persuade someone out of a principlist view – and I believe it’s often vital that we do – we have to find some other way. What is that way? I don’t know. But that’s the thought I’ll finish with for now.