In today's Guardian, David Cameron commits one of the most atrocious non sequiturs I've ever seen.

He begins from a reasonable premise - that "wider problems in the global economy pose a real risk to our recovery at home": emerging markets are slowing down; the euro area is close to recession; and uncertainty in Russia and the middle east might also depress growth. But then comes this:

As the global economy faces greater uncertainty, it is more important than ever that we send a clear message to the world that Britain is not going to waver on dealing with its debts. This stability is vital in attracting the business and international investment that delivers growth and jobs, and which keeps long-term interest rates low.

This is precisely the opposite inference we should draw if we believe, as Cameron does, that "red warning lights are once again flashing on the dashboard of the global economy."

If the world economy slows, exports won't pick up the slack created by restraint in public spending. And nor will investment, because uncertainty usually depresses capital spending.

What's more, uncertainty reduces gilt yields because investors seek out safer assets; it is this, more than confidence in fiscal policy, that explains why long-term interest rates are low*. This makes government borrowing more affordable.

The claim that there are big troubles in the world economy is a reason to support a looser fiscal policy, not austerity.

Nor is it adequate to claim, as Cameron does, that we should use monetary policy rather than fiscal policy to support growth. It's likely that quantitative easing - which is of dubious merit anyway - would be less effective now that it was in 2009, for two reasons:

- Gilt yields are lower now than they were then. This means they have less far to fall in response to Bank buying.

- Tail risk for the UK economy is probably smaller now. This blocks one of the mechanisms whereby QE works - its signal that the Bank is determined to avert the risk of catastrophe.

As a defence of austerity, Cameron's argument stinks. A far better defence would be to play down the risks to the world economy - eg by pointing to the likelihood of decent US growth and of the ECB doing enough to stimulate the euro area - and claim that the economy is strong enough to withstand austerity.

You might think that what Cameron is doing here is simply preparing his excuses in advance. If the UK economy does slow down, he can use the excuse of wimpering schoolboys down the years: "a big boy did it."

Even this, though, won't wash. As I said when Vince Cable tried to blame weak growth in 2012 on the euro crisis, a policy that works only if everything turns out well is not really a policy at all. Surprises are inevitable, and a good policy - in government, business or investing - is one that is resilient to bad surprises. Adverse developments overseas, then, are no defence whatsoever of austerity.

To be honest, I'm rather embarrassed to point all this out, as it is so trivially obvious. However, in our hyperreal economy in which "media macro" has displaced basic economic logic, arguments such as this feeble effort from Cameron are in danger of achieving a credibility they don't deserve.

* I say this simply because since June 2010 bond yields have fallen around the world.