Matthew Parris in the Times raises a challenge for Tories - but not, perhaps, the one he thinks he does.

The government's willingness to borrow to get through the crisis, he says, undermines the traditional "Conservative case for prudence in public spending." And if there are not the limits the Tories thought on public spending, then:

The Tories had better brace themselves serious questions about how we can do it for a virus but not to save a shipyard, or the planet from climate change. Or double the future capacity of our hospitals, or legal aid limits, or nurses and carers' wages.

I think Tories have - and always have had - two answers here. But they contradict each other.

First, let's dispose of bad reasons for "prudence."

In part, it is a legacy from different times. In the relatively closed economies of the 70s and 80s, there were limits on how much governments could borrow, set by inflation and savers' reluctance to lend to the state. Although these limits haven't been binding for a long time, it's common for people's ideas to outlive the reality behind them.

Also, the idea that government debt is a burden on future generations is wrong, and not just because they can roll it over to the next generation and because interest rates are negative. There's plenty else that can impose a burden on our children. Recessions have scarring effects, both by reducing the skills and experience young workers acquire, and by making people more risk-averse (pdf) in later life. Equally, poor education and inadequate infrastructure depress people's incomes in future. And a lack of action on climate change might be catastrophic for our grandchildren. Reducing the "burden on future generations" might as well justify increased government borrowing as lower.

Why, then, have the Tories had an instinctive (if not always realized) desire to limit public spending?

Simple. It's because they want a large private sector. If there were no moral or prudential limits on government spending, they fear, the state would expand at the expense of the private. Voters do not instinctively favour a market economy.

One reason why the Tories want a large private sector, of course, is that they are the party of capitalism, and capitalism is about making profits.

This explains why their belief in "prudence" has waxed and waned. Sometimes - as in the early 80s - austerity is needed to depress wages and so relieve the squeeze on profit margins. But at other times, such as the early 90s and now, capitalism requires increased public spending to shore up legitimacy, compensate for weak private sector demand, or provide the infrastructure that capitalism needs.

But there's a second reason why Tories want a thriving private sector. It's that this can sometimes deliver the goods.

The USSR put a man into space but couldn't give its citizens comfortable shoes. Which shows the strength and weakness of the state. It's good at achieving single agreed-upon objectives such as winning wars or (we hope) defeating viruses: there are no libertarians in foxholes. But it's bad at delivering a variety of goods or at innovating to improve the quality of those goods. For this, we need a market economy.

It's not just material goods that a market economy delivers, some Tories think. Following McCloskey and Montequieu, some believe in "doux commerce" - that commercial society inculcates virtues. They believe that success in markets requires an understanding of what others want, sobriety, prudence and the cultivation of good reputation.

Herein lies the real problem for Tories. These two cases for a large private sector can conflict with each other. "Doux commerce" requires constraints upon capitalists. As Jesse Norman writes in Adam Smith: What He Thought and Why it Matters:

Markets are sustained not merely by incentives of gain or loss, but by laws, institutions, norms and identities, and without those things they cannot be adequately understood.

Profit maximization, however, sometimes requires that these institutions and norms be ripped down, the better to pursue rapacious exploitation, monopoly, and cronyism. You can interpret that much-abused term "neoliberalism" as the process whereby "doux commerce" is supplanted by a harsher form of capitalism - in which we lose the James Timpsons and get the Mike Ashleys.

In this process, says Jesse:

Business activity loses any relation to, and often clashes with, the public interest;...business merit is separated from business reward. These features then feed off and into a culture in which values of decency, modesty and respect are disregarded, and short-termism and quick rewards come to dominate long-established norms of mutual obligation, fair dealing and just reward.

It's not just in the realm of morality that there's a conflict between capitalism and commercial society, however. There's also a conflict in economics. A genuinely thriving free market would see few concentrations of private wealth or power, because competition and creative destruction would eliminate them. American leftists have the slogan "every billionaire is a policy failure", but it would be equally true to say that every billionaire is a market failure.

Herein, I suspect, lies the biggest dilemma within conservative thinking. It does not concern their attitude to "prudence" in the public finances: it is sometimes correct to suspend such prudence. Instead, their problem is: why do they want a large private sector? Because there is a conflict - which has grown markedly in recent years - between a healthy commercial society and more exploitative forms of capitalism.

We might wonder why this dilemma is not more publicly discussed. One reason, I suspect, is that here we have another example of an idea outliving their material base. In the mean and nasty 70s, it was plausible that there was little tension between these two conceptions of a market economy; one could believe that more capitalism would mean doux commerce. It is, however, less plausible now. Another reason, perhaps, is that the Tory party has, in one respect at least, an emergent intelligence that Labour lacks: it knows instinctively that some issues should not be raised.