The biggest threat to capitalism is not Marxists but capitalists themselves. That's the thought which struck me from reading Ann Pettifor's claim that a Corbyn government could be good for business.

I'm thinking here of two analytically separate mechanisms.

The first arises from a simple identity, that the aggregate profit rate can be increased in one of two ways - either by increasing the share of profits in output, or by raising the output-capital ratio. Thatcherism tried the former - for example by weakening workers' bargaining power. Post-war social democratic Keynesianism tried the latter: it hoped that full employment would keep the capital-output ratio high and hence maintain profit rates and therefore incentives to invest,

Ann's point is that a Corbyn government would also try the latter; looser fiscal policy would raise the output-capital ratio and hence profit rates. "Falling incomes and spare capacity have not been good for business" she says.

The second mechanism lies in the fact that the success of capitalism rests upon two things: accumulation and legitimation. On the one hand, capitalists need high profits and incentives to invest such as a favourable tax regime. On the other hand, though, the system must possess a minimum of legitimacy so that people support it.

These two conditions can sometimes conflict. Take, for example, high house prices. On the one hand, these help sustain capitalism. High rents and mortgages force people to work long hours thereby giving capital a quiescent workforce. As David Harvey writes, "barriers of private property in land...[are] needed to ensure an adequate supply of exploitable wage labour for capital." On the other hand, though, if people feel they have no chance of acquiring property, they'll see no hope of getting a stake in the system and this will undermine its legitimacy. As Thatcher recognised, a property-owning democracy shores up support for capitalism.

High inequality can also undermine legitimacy. If people feel the rich are ripping them off - for example by not paying their "fair share" of tax - there'll be a backlash against capitalism.

Intelligent Tories - yes, there are some if you look hard enough - recognise this problem. Sebastian Payne in the FT quotes Nick Boles:

Roosevelt's key insight was that capitalism in the 1890s in the US was becoming its own worst enemy," he says. "It had been captured by robber barons and monopolistic groups. It was destroying the interests of the small businessman, the small farmer and the consumer. He worked out that if those who believed in capitalism didn't reform it, then the populists would sweep it away."

Again, Labour might be the saviour of capitalism here. Its plans to reduce house prices, raise the living wage and increase corporate taxes might increase the legitimacy of the system.

From this perspective, we can regard the function of the state in capitalist society as being to solve a collective action problem - the fact that what is in the short-term interests of individual capitalists might not be the longer-term interests of capitalists as a class.

For example, each individual capitalist wants wages to be as low as possible, compatible with sufficient recruitment and motivation. But if wages are low, the market for consumer goods will be weak*. Or each individual capitalist might want tax breaks and rigged markets in his favour but these undermine the legitimacy of the system. (This might be the eventual effect of Trump's tax plan.)

It's possible that a Corbyn government would solve these collective action problems.

But only possible. It's not clear that there is as much spare capacity as Ann thinks: the OBR and Bank of England think there isn't. If there's not, then looser fiscal policy might lead to higher interest rates rather than a higher output-capital ratio. Maybe higher corporate taxes would lead to more tax-dodging or less investment. Or perhaps a higher wage floor would, at some point, cost jobs. And wage-led growth might not work.

The difference between Marxists such as me and social democrats such as Ann is, I suspect, that we are more sceptical than she is of governments' ability to fix capitalism.

This, though, is a comradely debate for another day. My point here is that it's possible that Labour will save capitalists from themselves. The narcissistic over-entitlement of the stupider capitalists, however, means they cannot see this.

* This dilemma can be overcome by allowing workers greater access to credit, as Thatcher did. But this might not be a permanent fix.