If the Labour leadership election is anything like the General Election, it will be completely uninformed by sensible economics. This is to be regretted because there's one fact which - if it continues - must shape leftist politics, namely the stagnation in productivity growth. This is such an important issue that even the BBC can no longer ignore it (1'13" in).

My chart shows the point. Output per hour is now 14.8% lower than it would have been if had grown since 2007 at the same rate as it had in the previous 30 years, of 2.3%. This is a massive gap - so much so that it is unlikely to have a single explanation. And we shouldn't expect it to be narrowed soon. The Bank of England yesterday predicted a pick-up in productivity growth but it foresees growth of only 1.7% in 2017 - below its pre-crash trend. And Deputy Governor Ben Broadbent warned that this forecast is "tremendously uncertain."

If productivity growth remains weak, it would have at least three political implications:

First, it means average GDP growth will be low. This is because it could only grow if employment expands. But the Bank is already worried that employment is so high as to threaten to raise nominal wage growth and hence inflation - which means that further incipient employment growth would lead to higher interest rates which would choke off growth.

This implies that the old assumption of both Tories and New Labour - that the economy would grow if only the right policy framework is in place - is mistaken. Supply-side policies - industrial policy if you like - are imperative. As for what these should be, there are many possibilities, including a more liberal immigration policy. I suspect we need a broad spectrum.

Secondly, if the economy's not growing much it becomes harder to fund public services; you can do so only by running a bigger deficit or by imposing higher taxes - and if real incomes are stagnating then resistance to the latter is likely to be severe.

Thirdly, stagnant productivity means that real income growth becomes a zero-sum game; workers can only see real wages rise if real profits fall, or one group of workers can only gain at the expense of others. In this context, raising "aspirations" would be a recipe for conflict.

All of this provides both challenges and opportunities for the next Labour leader.

The challenge is that Labour's traditional promise of funnelling cash towards health and education will be difficult to deliver.

The opportunity is that serious supply-side policies would offer something to all workers, not just the low-paid; this would be especially true if such policies include - as they should - greater worker ownership and control. Labour could claim that only it has a way of raising productivity and hence real wages; Owen is right - "aspiration" can be harnessed by the left.

The big issue in the leadership election should be how about these challenges and opportunities are addressed. I fear, however, that Paul Mason might be right, and the party lacks the intellectual resources to do so and what we'll get instead are empty words.