The Stink That Saved London

In the summer of 1858 the Thames became so foul that members of Parliament, meeting in a building on its bank, hung curtains soaked in chloride of lime over the windows and debated whether to move the government elsewhere. Heat had thickened the river's load of household and industrial waste into a stench that newspapers named the Great Stink. Within weeks legislators who had spent years quarrelling over the cost of a sewer system passed the necessary law and authorised the engineer Joseph Bazalgette to build one.

The reason for their haste was a theory. Most physicians of the day held that epidemics such as cholera were caused by miasma, a poisonous vapour rising from rotting matter. A river that smelled of death seemed to threaten the capital with disease, and the lawmakers who breathed it had the strongest possible motive to act. Bazalgette's network of intercepting sewers carried waste eastward and released it into the estuary far below the city.

The theory was wrong. Four years earlier the physician John Snow had traced a cholera outbreak in Soho to a single contaminated water pump and had argued that the disease travelled in water and not in air. Most of his contemporaries were unpersuaded. Yet the sewers worked against cholera exactly as Snow's account would predict, since they removed sewage from the water that Londoners drew and drank. The last major outbreak in London, in 1866, struck chiefly a district whose water supply was still contaminated.

It is tempting to conclude that the accuracy of a theory matters little when the action it inspires succeeds. A mistaken belief had delivered the right remedy. Defenders of miasma could even claim vindication, since foul smells and dirty water tend to occur together and clearing one cleared the other. Nothing in the outcome compelled anyone to abandon the old idea.

A mistaken belief had delivered the right remedy.

That partial vindication is precisely the danger. A theory that produces good results for the wrong reasons will fail unpredictably when its hidden assumption stops holding. Had the sewers been designed to ventilate odours and not to remove waste, or had the smell been masked with chemicals, the theory would have recommended measures that left the water untouched. Snow's account could say in advance which measures would work, while the false one only happened to point the same way.

Success, then, is a poor judge of the ideas that produced it. Every policy that works invites gratitude toward whatever reasoning came before it, and that reasoning may be sound or merely aligned with the truth by accident. It remains unclear how anyone could know, in the moment of triumph, which kind of luck they have enjoyed.