It was the question behind ‘the project’ – the fabled plan to reconcile the centre and left traditions in British politics. How do we resolve David Marquand’s ‘progressive dilemma’ – the failure of Britain’s non-Conservative forces to see their combined support reflected in representation?

The historians will chew over the tactics of what happened after 1997. Some will argue that the Liberal Democrats and Labour missed a chance of working together. I personally think it was the Liberal Democrats who missed the chance of occupying the centre right and pushing the Conservatives into an ideologically authentic, but politically landlocked, Thatcherite position.

But there was a more fundamental reason that a progressive coalition did not emerge after 1997: it was that there was no progressive consensus. The Liberal Democrats have been profoundly unsure about whether they agree with New Labour’s approach – whether over choice and contestability for public services; a responsibility to work as well as a right to welfare; being as tough on crime as on the causes of crime; or liberal interventionism in foreign policy.

The Liberal Democrats have been a bit all-over-the-place on these issues: in Labour’s first term, they were broadly supportive, but wanted higher taxes; in the second term, they parted company with us on foreign policy, but also on the crime agenda and choice in public services. Recently, they have swung back the other way, trying out ‘tough liberalism’, flirting with social insurance in health and now even purporting to want to cut taxes.

So it’s hard to say what the core Liberal Democrat ideas are. But one thing is clear from the last 10 years: there has not been unity between our parties about these methods. No progressive coalition has been possible, even if it had been desirable, because no progressive consensus existed.

So, rather than discussing the topography of British politics, we need to debate the fault lines. Rather than talking tactics, we need to debate ideas.

This debate needs to be based on engagement, not triangulation. Triangulation positions – it doesn’t convince. It sets up false choices – our left-wing critics would do this peculiar thing; our right-wing critics would do this bad thing; so the only option is to do our reasonable thing. By definition, no one can debate false choices.

If those who disagree with us do not feel we are representing their position fairly or accurately, they will not engage with our arguments, either. We will fail to convince them when we’re right, and fail to hear them when we’re wrong.

Of course, political parties will often disagree fundamentally and attack each other’s positions. But that shouldn’t crowd out the space for engaged debate on other issues. So we should have the confidence to argue not with the worst depiction of others’ positions, but with their best formulation. Discussion with a strong opponent can only improve our own position.

This candid, rounded debate is particularly important within the Labour party because we face a real decision about our direction of travel. On public services, for example, we face decisions on at least three core debates. First, what is our modern definition of Labour values? Is equality predominant, or is autonomy as important? Second, can diversity of quality be countenanced? Third, what role is there for choice and contestability?

The New Labour view is that autonomy is a core left-wing value – redistributing power is as important as redistributing income. We believe that diversity of quality is inevitable, and that choice is a key tool – to deal with under-performers, increase the supply of better providers, and widen access to them. In other words, choice and contestability can improve the quality of services and reduce inequality of access.

These are contested arguments. They turn on evidence and on an assessment of how they deliver our values. There are areas where we have changed our mind – for example on the extent to which we should use targets and inspection. There will be more in the future.

But overall, we are confident of our approach. If we are confident that these arguments are correct, then we should be confident about debating them. We should be clear about our reasons for having these views, because otherwise we won’t convince people that the direction of travel is a desirable one. If we are right, yet fail to convince our critics, the penalty will be paid by the poorest in society who will fail to benefit from us continuing those policies, and indeed taking them further. And, unless we convince people of the strength of our ideas, there will be little point even discussing the progressive dilemma.