... A recent lawsuit in New York, brought on behalf of American blacks is structured in just this way. It seeks to recover money from banks and insurance companies that, the plaintiffs argue, assisted slavery. It wants them to cough up for slave-generated profits from before 1865 that have allegedly been invested in their businesses since.

Slavery in America exacted a terrible human toll. For a long time most economists thought that slavery was not just bad, but also inefficient. Then in the 1970s, an economist at the University of Chicago, Robert Fogel, estimated that the economies of America's slave-holding states were actually 9% more productive than those of free states, work which later won him a Nobel prize. Although morally appalling, slavery turns out to have been good economics.

But what was the financial cost borne by the slaves themselves? One measure is the difference between slaves' wages and those of free workers with the same skills. Mr Fogel reckons that, between 1780 and 1860, just before the American civil war, slaves were paid (in food, shelter and so on) around 10% less than similar free workers. He estimates a cumulative bill for slaves' expropriated wages of $24 billion in 1860.

Compound interest on this sum for 142 years has a massive effect. A risk-free interest rate of 6% a year, which is what Mr Fogel estimates is the long-term rate, brings the cost to $97 trillion, more than nine times the size of America's economy today. Awarding interest of just 3% a year would cut the total bill to $1.6 trillion, not far from damages cited in the current lawsuit. These figures are merely for lost wages. They do not take into account other pain and suffering caused by slavery, which is harder to calculate.

If the principle of reparations were ever agreed upon, what of ascertaining who were the beneficiaries of slavery? Slave-owners make the likeliest targets, yet only 5% of American households had slaves. What is more, owners received only a small fraction of the benefits from slaves' forced labour. Most of the gains, Mr Fogel reckons, were passed on to consumers in Europe (and England in particular) in the form of cheap cotton and tobacco. Mr Fogel suggests that reparations are, at least in part, “a debt for the European Union”.

... Mr Fisher argues that, as a measure of damage, slaves' lost wages are only a bare minimum, since they do not reflect the higher price a person would require freely to choose a life of bondage. Indeed, it would be hard to estimate any market price for enduring the forced separation of families that marked slavery in America.

Arguments about reparations do not end there. Some point to the 350,000 Union troops who died in the war that ended slavery, and ask what compensation their descendants are owed. Others argue that, for all the degradation of slavery, without it slaves' descendants would not now be enjoying much higher prosperity in America than they would experience in their ancestors' homeland. Any reparations should reflect this.

All such calculations may be moot, since such huge figures are unlikely ever to be paid. Moreover, the human costs of war, terrorism and slavery seem to defy any financial calculation. Perhaps on these big questions, economists should leave the field to philosophers and poets—and even politicians.

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