All money tends to corrupt, and absolute money corrupts absolutely. This is an ancient message. You can find it in the Bible ("the love of money is the root of all evil"), in the writings of ancient Greek philosophers and Renaissance moralists, and more recently in the Occupy movement that set up camp last year outside St Paul's Cathedral. This Wednesday, the cathedral was packed for a rather more sedate explanation of the same ideas  featuring the Harvard philosopher Michael Sandel.
Sandel, currently plugging his new book What Money Can't Buy, has been difficult to avoid in recent days. His central thesis is twofold. Firstly, when you put a price on something you alter its intrinsic properties, and this can be morally corrosive. Secondly, the past few decades have seen a market economy replaced by a "market society" in which "everything is up for sale". Markets, he says, "are not neutral instruments, they crowd out values worth caring about" - values like altruism, human dignity and the common good. As a result we have seen a great hollowing-out of communality and public political discourse.
He asks such questions as: is it right to create a market in blood, rather than rely on altruistic donors? Should unhealthy people be given financial incentives to adopt healthier lifestyles? Should school pupils be "bribed" to read books or achieve higher marks? To all these questions Wednesday's audience answered an emphatic "no", which suggests that Sandel is, at least in terms of public opinion, pushing at an open door.
This may explain the tremendous popularity he now enjoys. (The Guardian described him  the other day as "currently the most effective communicator of ideas in English" and suggested that his latest book "should be the bedside companion of every Miliband aide".) The free market "experiment" of the past few decades has led to rising inequality and an economic disaster, the only beneficiaries of which would seem to be a handful of already wealthy bankers. We should not be surprised if Sandel's deeply traditional complaints about the corrosive effect of money on the human soul find a ready echo, especially when voiced in a cathedral whose history and location give it a somewhat ambiguous relationship with wealth.
The idea that money has destroyed all vestige of civic virtue was hackneyed already in Roman times. For all Sandel's current vogue on the progressive Left, his message is inherently a conservative one, in that it implicitly looks back to a Golden Age before money ruined everything. Another way of saying this is that there's nothing new about the "market society".
One of Sandel's examples relates to privately-run prisons in California in which convicts with sufficient means can upgrade to a better cell. This was standard practice in 18th century London. Also popular in the 18th century was the "tontine", a form of gambling in which a group of people pooled their resources and the last one left alive collected the jackpot: not too dissimilar, in essence, from the market in third-party life insurance that Sandel criticises today.
But then to talk about the 18th century is to realise just how much more thoroughgoing the marketisation of society used to be. From the horrors of the slave-trade and the near-slavery of indentured labour, to the open purchase of Parliamentary seats through "rotten boroughs", almost everything was up for sale. Commissions in the British army and civil service appointments were bought, rather than given on merit, well into the 19th century. What we think of as basic public services such as policing and the upkeep of roads were wholly private or at best put out to tender. And it's unlikely to be a coincidence that prostitution in the 18th century was vastly more extensive and exploitative than anything seen today.
The present-day "market society", for all its deficiencies, is a pale shadow of the ruthless and money-driven world of two or three centuries ago. Sandel is squeamish about students hiring out their foreheads to advertisers or paying homeless people to stand all day in queues so that a richer and busier person can get into Congressional hearings. There used to be an actual trade in human beings. Things aren't likely to get that bad again, however badly things go in Greece.
At least when something has a price it shows that someone puts a value on it. Not charging for goods or services can lead to problems of a different order. The BBC's Stephanie Flanders, taking part in the debate at St Paul's, pointed out that in the age of the internet, many goods and services which would in the past have been paid for are available for free. The thought struck me that perhaps not charging for a service, or expecting things to be free, can be at least as morally corrupting of basic goods as Sandel believes money is.
If people expect to, and can, receive their news and entertainment for free, why should they pay for it? And how can the producers make an honest living? The Bank of England's Andrew Bailey contends that free banking distorts the market, is less transparent and leads to poorer service to consumers. It is at least an arguable case. And as regards to "free" internet services like Google and Facebook, it has well been said that the non-paying users are not the customers, but are themselves the product.
Is money the source of the problems Sandel identifies, or rather a convenient scapegoat for human beings who can't bear too much reality? You can't buy a friend, he points out, because if you know you've paid someone to be nice to you it ceases to be a "real" friendship. Has he never noticed that rich people tend to have more "friends" than poor ones? Sandel also raised the example of a professionally written wedding speech. Would the bride and groom feel quite the same way, he wondered, if they knew that the best man had spent $150 dollars on buying a speech rather than investing his heart and soul by writing it personally? Perhaps not, but it's not obvious to me why the payment of money in itself is corrupting.
The problem, surely - if there is a problem - is that the speech is not the best man's own; not that he has paid for it. I rather doubt that the newlyweds would be happier to learn that the best man had found the speech on a website and simply downloaded it for free.