Richard Murphy touches on an issue which, though neglected, divides political activists from many observers, and social democrats from Marxists. He says: "[when] it comes to fairness majority opinions matter."

Now, this is true in the sense that a society whose policies and institutions violate the majority's perceptions of fairness will be an unstable one - as nation-builders often discover.

Where it is doubtful, though, is whether majority opinion decides what's fair.

There's a long tradition in ethical thinking, from the book of Exodus - "Thou shalt not follow a multitude to do evil" - to Amartya Sen (pdf) and some interpretations of intersectionality which doubts the ability of the majority to decide what's fair.

Most of us would accept these doubts in the context of other societies. We don't think the repression of women and gays is fair in Saudi Arabia, nor that it was fair in the past, merely because most people in Saudi Arabia or in early 20th century England thought them so. The statement: "slavery was fair once" would strike most of us as absurd, or at least as requiring a lot of justifying.

But why should we suspend such doubts in today's Britain? Most people - to take one example - agree with the statement: "it is fair that foreigners should be excluded from much of the UK labour market." But a few decades ago, most would have agreed with the statement: "it is fair that women should be excluded from much of the labour market." What, morally speaking, is the difference? Could it be that we see one merely because Peter Singer's "expanding circle" of concern has expanded to include women but hasn't yet expanded to include foreigners?

Let's face it, the standard of public "debate" about moral questions is not high; most claims about morality are mere emotivist spasms or expressions of narcissistic self-righteousness.

I suspect - though this might be the confirmation bias! - that recent thinking has deepened scepticism about the ability of the majority to perceive what's fair. Cognitive biases such as the just world fallacy, stereotype threat, adaptive preferences and anchoring effect help to provide popular support for inequality; John Jost calls this system justification (pdf).

It's in this context that there's a difference between Richard and me. He says:

On the welfare cap I have no doubt the majority will consider what is being proposed to be profoundly unfair if they realise just who is affected.

But I don't much care what the majority thinks; the cap is either fair or not, regardless of what the majority think.

This difference reflects two differences between us. One is that Richard's a social democrat and I'm a Marxist. Whereas social democrats try to work within the confines of what the public considers "fair", and try to tweak those perceptions, we Marxists fear that this is a forlorn task because the power of ideology warps those perceptions.

The other is that Richard is an activist and I'm an observer. And in a democracy, success as an activist is determined by what the majority think - by whether you win elections; how often do we see politicians cite opinion polls as if they decided matters?

And this is what bothers me. Public opinion might decide what is a successful political strategy, but it is more questionable whether it should decide what is a morally right one. One of my fears about Labour politics is that this distinction is often ignored.