Last night, I saw a good example of how the non-Marxist left and the right can both miss something important.

Zoe Williams complains that Richard Branson is trying to monetize the mega-rich's desire for the consumer experience of space travel. Ryan Bourne replies that this is an "age-old critique of capitalism that has been proved wrong" and that capitalism has delivered a massive increase in living standards.

What this misses is that Mr Branson isn't putting anybody into space; Saturday's crash was the latest in a long series of setbacks for Virgin Galactic. The private sector is struggling to do something that a centrally planned economy achieved 53 years ago.

That's a cheap shot. But it's consistent with a wider and more important trend, embodied in talk of secular stagnation and an end (pdf) to growth - that capitalism has lost its dynamism lately. We see this in several ways:

- Real wage growth has been trending downwards in recent years.

- Companies in the US and UK have huge cash piles, in part because they don't see opportunities to invest profitably in real assets.

- Long-term real interest rates in the west are negative. Granted, this might be partly due to a safe asset shortage or global savings glut. But it might also be a sign that investors have no confidence in economies' abilities to grow.

- The growth of crony and casino capitalism are symptoms of a lack of productive investment. Unable to make money by innovating and expanding, capitalism turns instead to trying to capture the state or to financial speculation.

Perhaps, therefore, that Virgin Galactic's failure is a symptom of a wider and deeper failure of capitalism.

It's for this reason that I say both Zoe and Ryan are missing something.

Zoe is missing the possibility that the problem with capitalism isn't merely one of morality, but of efficiency. Feudalism collapsed not because of its immorality but because of its inefficiency. Why should capitalism be different?

Ryan, on the other hand, is making the same mistake Bertrand Russell's chicken made in expecting the farmer to feed him when in fact he breaks his neck; he's assuming that what's happened in the past must continue. Yes, he's right that capitalism has produced a massive rise in living standards - though we might note that the increase was greater in the post-1945 social democratic period than in the pre-1914 golden age of free markets. But the drop in real wages in recent years in the US and UK calls into question whether this can continue.

Might it instead be that Marx's prediction is coming true?:

At a certain stage of development, the material productive forces of society come into conflict with the existing relations of production...From forms of development of the productive forces these relations turn into their fetters.

Now, I raise this as a question to which I don't know the answer. But it's a question that both critics and supporters of capitalism overlook - that what is (or should be) at issue isn't merely the morality of capitalism, but its effectiveness in raising living standards.