ON Wednesday night I was asked a question I’ve never been asked before. James Cook, who took on the tough job of chairing the BBC Scotland leaders’ debate from Aberdeen, challenged all parties to lay down “red lines” for negotiating with others in the event that a balanced Parliament is formed at Westminster next month.

It was a perfectly fair thing to do. Voters have the right to know which policies are up for compromise, and which are unshakable principles; just think for example of the 2010 betrayal of the student vote when the Liberal Democrats agreed to increase tuition fees to £9000 after signing a high-profile pledge to do the opposite.

There are some easy examples. Trident; I’d never support that. Saving the Union; that’s an obvious one for Ruth Davidson. But here was the question James threw at me – for Greens, is capitalism itself a red line?

It’s a deceptively simple question, and almost as soon as it was asked I was surprised nobody had ever asked it before. Mildly startled, I’d have to admit my answer could have been clearer.

There’s a left-right spectrum within most political parties, and the Greens are no exception. Some of us are more comfortable than others with an explicit “socialist” tag. Some are more comfortable than others with a mixed economy. But a rejection of the dominance of big business in much of our economy pushes the Greens towards a bigger role for shared ownership – both public and community – as well as an appetite to see small, independent businesses prosper.

Oddly, the deeper question – is capitalism itself sustainable? – isn’t often subject to debate either within the Green movement or outside it. Most other political parties, even those born out of a desire to challenge capitalism, have now reached an accommodation with it and offer only to manage it better. Most Greens, on the other hand, regard capitalism as an economic system which takes relentlessly from the global commons and contributes little or nothing back to them. In short, it is too effective at extracting value, doing so by exploiting people and the environment, to be called sustainable.

Resources are overharvested, from fish stocks to hardwoods, because our economic system attaches value to their short-term exploitation and fails to attach value to their long-term stewardship. Workers are underpaid or put at risk because the quality of their lives doesn’t matter to the market. Pollution of land, water and air continues apace, because the processes which create it generate profit for someone, but the consequences don’t show up on the balance sheet.

Even when a market develops for ethical or sustainable business practices, it rarely achieves more than a niche alongside the unethical and destructive practices. So renewable energy is installed, but fossil fuel companies and the national economies which depend on them carry on extracting all that buried carbon, most of which will end up in the atmosphere, or in the waste materials which litter the world. Even if renewables take a bigger role in the energy mix, as overall consumption grows the damage to the climate accelerates.

There are those Greens who make the case that whether we like it or not capitalism is here to stay, at least for the time being.

If it’s the only game in town, they argue, we have no choice but to make it operate within the social and environmental limits which will leave us with a liveable world. I think that argument walks a dangerous line between delusion and defeatism. It suggests that we should pass up opportunities to develop better economic models on the tenuous assumption that capitalism is capable of the fundamental change that’s needed.

However, it would be absurd to imagine that any one government could wish away capitalism and the constraints it imposes – far less so in a single term of office. In saying that capitalism is unsustainable, I don’t mean that it will be brought down with placards and slogans. It will end because it must.

The only question is what economic model can replace it. There are I believe already glimpses of a new economic model rising, one based on far more lateral, peer-to-peer economic relationships and in which power might decentralise.

So is capitalism a “red line”? I don’t think it works that way. To give a simple answer would seem like a refusal to engage with the world as it is. Capitalism is a dominant force in our lives, and that reality can’t be overcome by any single party or government.

I cannot see the Scottish Greens ever working closely with its uncritical cheerleaders. But if any party is capable of seeing its shortcomings, and working with an open mind toward a better kind of economy, then that sounds like common ground worth reaching.