St Paul’s Cathedral was packed this week to debate whether there was anything that money couldn’t, and shouldn’t, buy.

Professor Michael Sandel spoke to the audience of more than 1,500 and probed the limits of what markets could achieve. Introducing the debate, Ann Pettifor referred to this as one of the most important issues of our time, and insisted on the need to go further than just criticising greed.

The central question for the discussion was set out by Sandel at the start: "What should be the role of money and markets in a good society." Sandel is taking part in a series of events across the UK to promote his new book What Money Can't Buy.

Sandel set out a series of examples where money was playing an increasing role in areas where previously it had not. This ranged from prisoners able to pay for an upgraded cell, to students renting out their foreheads for adverts for £4.20/hour.

"We have drifted from having a market economy to becoming a market society," Sandel said. "A market economy is a tool for organising productive activity, but a market society is a place where everything is up for sale."

While there are some things, Sandel explained, such as friendship that money cannot buy, there are gifts of friendship that can be bought. For example, a wedding speech can be professionally written for $149 (£95), key facts are plugged into a web form and three days later it's delivered. Are the words of friendship that you offer to the happy couple, which may move them to tears and then to laughter, devalued because you did not write them yourself?

A further example which proved controversial in the ensuing discussion was payment to children for writing thank you notes. Would the practice of writing notes instil in the children good behaviour that would continue if payment would cease, or do the notes stop when the money stops? Is the act of paying for the notes making it harder for children to learn the virtue of gratitude?

Responding to Sandel, Julian Le Grand acknowledged that there were sometimes harmful affects of market influence in the trading of non-traditional good, but suggested that other ways of allocating resources also came at a cost.

Bishop Peter Selby, the former bishop of Worcester, suggested that Sandel was providing a secular warning against idolatry, about what happens "when a useful thing starts to extract from humans a delight and honour that it does not deserve".

BBC economics correspondent Stephanie Flanders recounted from her own experience of encouraging children to do things that incentives were sometimes essential. Was it somehow worse to offer money for the completion of a good task than chocolate? She went on to say that this debate was "a good reminder that we can't sub-contract moral questions to the market" and went on to accept that "real answers are messy".

In his concluding remarks Sandel discussed the option of paying for blood supplies, and whether this undermines the very activity of giving. Is it, for example, still a good thing to give blood voluntarily, if in doing so you are depriving someone else of a role in the labour market?

"The deeper hold of markets is that they seem to offer a way of avoiding a public judgement of the meaning of goods," Sandel said. "After all an exchange between consenting adults spares us from entering into deliberations as to whether the exchange has affected the nature of the good."