Fisher's Chest: An Inquiry into the Nature of Causes, Health and Caution

Man has always been learning that correlation does not imply causation. When expulsed from the Garden of Eden, Adam had acquired from the Tree of Knowledge the understanding that he could by no mean assert that the temporal correlation between his apple biting and his expulsion from the Garden of Eden was a causal one. Since then, each time and civilisation has developped its own way to state and teach the principle, so much that the evolution of this instruction can be seen as a satisfactory proxy for the study of human history. It is therefore difficult to understand why so little attention has been paid so far for making an history of the teaching of the primary principle of statistics. It was in the attributes of this blog to fill the gap.

What causes people to discover that correlation does not imply causation? The generation to which I failed to belong learnt it the hard way. As for the second half of the XXth century, most people were then taught that being black was not necessarily the cause of poverty, since one could not reject the reverse causality, poverty turning people black. At the beginning of the XXth century, Professor Udny Yule told the story of a large significant correlation between the number of apples imported into Great Britain and the number of divorces.

This last exemple is cited by Ronald Fisher, probably the most important statistician of the XXth century, whose contributions to science seem however to have been highly selected in the freshman statistics handbooks. For behind the test is an incorruptible man of principles, whose fights for science have been kept in silence. He is for instance the author of one of the best attempts to teach the public opinion with the difference between correlation and causality. His major contributions to the subject are gathered in his book Smoking, The Cancer Controversy, Some Attempts to assess the Evidence, which gathers many of his articles, most of them written as a reaction to the conclusions of Richard Doll and A.B. Hill on lung cancer, from which the passage below is taken :

The subject is complicated, and I mentioned at an early stage that the logical distinction was between A causing B, B causing A, something else causing both. Is it possible, then, that lung cancer – that is to say, the pre-cancerous condition which must exist and is known to exist for years in those who are going to show overt lung cancer – is one of the causes of smoking cigarettes? I don’t think it can be excluded. I don’t know enough to say it is such a cause. But the pre-cancerous condition is one involving a certain amount of slight chronic inflammation. The causes of smoking cigarettes may be studied among your friends, to some extent, and I think you will agree that a slight cause of irritation – a slight disappointment, an unexpected delay, some sort of a mild rebuff, a frustration – are commonly accompanied by pulling out a cigarette and getting a little compensation for life’s minor ills in that way. And so, anyone suffering from a chronic inflammation in part of the body (something that does not give rise to conscious pain) is not unlikely to be associated with smoking more frequently, or smoking rather than not smoking. It is the kind of comfort that might be a real solace to anyone in the fifteen years of approaching lung cancer. And to take the poor chap’s cigarettes away from him would be rather like taking away his white stick from a blind man. It would make an already unhappy person a little more unhappy than he need be.

Fisher's book is available online, in particular this paper, whose title would be thought to be borrowed from a Jim Jarmusch's movie, if we did not know it was the reverse causation at play.