This post has been sitting around in draft form for a few weeks. It's from January 2, 2009; I'm reposting it now simply to make it visible on my blog.
Ignaz Semmelweis is a famous name in the history of medicine. This is his story, in brief, selectively paraphrased from Carl Hempel's Philosophy of Natural Science. Semmelweis's findings have been described elsewhere on Open Salon, but here we'll review how he reached his conclusions.
In the late 1840s, Dr. Semmelweis oversaw activities in the First Obstetrical Clinic of the Vienna General Hospital. His supervisory duties included teaching medical students and examining patients. He was troubled: the mortality rate for women giving birth in the First Clinic was very high, reaching 10% and even 15% in some years
. He was also puzzled: the adjacent Second Clinic, staffed with midwives rather than medical students, managed to keep their mortality rate below 3%. Semmelweis undertook a careful, systematic study to find out why.
He began by ruling out various explanations that were inconsistent with his observations and inferences. Could the problem be overcrowding? No. The Second Clinic was more crowded than the first. In fact, the First Clinic had become so notorious that women preferred to give birth in the street rather than be assigned there--and fewer deaths accompanied those so-called street births than births in the First Clinic. Could it be roughness in how the mothers were handled by medical students in the First Clinic versus by midwives in the Second Clinic? No. There were differences, but these paled in comparison to the trauma of the birth process itself. Could the problem lie in some general epidemic spread of disease? Semmelweis was working a few decades before the germ theory of disease was to become firmly established, and thus it made sense to think of bad air, a miasma, that might possibly affect women's health. But such a miasma would certainly affect people living in the city outside the hospital, and this was not the case.
Semmelweis also eliminated some possible explanations through experimentation. The most interesting of these had to do with a psychological explanation: A priest would visit the dead and dying in the Hospital; he could directly and quietly enter the Second Clinic but could only reach the First Clinic with some fanfare--an attendant preceding him through several rooms, ringing a bell. Could the fear and anxiety provoked by this practice lead to more deaths during child birth? No. A change to the priest's route and procedures had no effect.
In 1847, Semmelweis's colleague Jakob Kolletschka died; he had been accidentally cut with a scalpel during a dissection in the autopsy room. Semmelweis saw similarities between Kolletschka's death and those of the women in the First Clinic, and he made an important connection. It was common practice for medical students to move from the autopsy room to the First Clinic to do their rounds, washing their hands superficially if at all. (Midwives in the Second Clinic never came near cadavers, of course.) What if...? Semmelweis instituted a new regimen that involved hand-washing with a chlorinated lime solution, and the mortality rate in the First Clinic immediately dropped by a factor of ten.
Hempel, my main source for the information I've recounted above, uses Semmelweis's story to introduce the basics of the scientific method: a reliance on modus tollens, the nature of hypothesis testing, the importance of induction. I'm going to be a bit more expansive in my gloss of this piece of history, to touch on other issues in the philosophy of science.
The importance of existing theory. One of the difficulties that Semmelweis faced was the lack of an appropriate theory to apply in interpreting his observations. While he could recognize that some of his hypotheses were more plausible than others, he needed to test many, many more ideas than we would today. Every modern schoolchild is told that washing our hands eliminates germs--but we wouldn't know about this without the incremental march of scientific progress.
The importance of explanatory mechanisms. A closely related difficulty for Semmelweis was the lack of a mechanism for explaining his success. "Okay, fine, your proposal worked--but why did it work?" Science isn't satisfied with regularities, even predictive regularities; we want an understanding of causal relationships. (For an example from a completely different field, the lack of a plausible mechanism delayed acceptance of Alfred Wegener's ideas about plate tectonics for about half a century.) It wasn't until Pasteur's time that a sufficient theoretical framework was established to explain Semmelweis's results.
Distinguishing between causal and associative factors. Semmelweis was able to reject some explanations because they were inconsistent with what he observed, but others only by running experiments. One way to think about this distinction is that everything that we observe in a given situation is consistent with that situation, associated with it, but only a few factors can be thought of as causing it. It's possible, under some conditions, to draw conclusions about causality without so-called manipulation, but often a manipulation experiment is the most straightforward: we make changes to see whether some factor causes a specific effect--for example, by removing it and seeing whether the effect recurs.
Accounting for chance. Semmelweis was lucky in one respect: his change to how medical students washed their hands produced a dramatic effect. What about a situation in which some change causes a small effect? When are we justified in concluding that an experiment has given us some insight into a causal relationship? This is part of what's sometimes called the logic of hypothesis testing. Imagine that Semmelweis has run an experiment and seen mortality reduced from 12.5% in one month to 11.0% in the next. Ignoring all his other sources of information (e.g., symptoms), he might say, "But this amount of variation is consistent with what has been observed over the past few years--it might just be due to chance." In general, if chance could have plausibly produced the same effect as some action we've taken, we refuse to attribute that effect to our action. After all, it could have come out differently.
Now, not all of us are scientists. But there's interesting evidence that some of what we do in our everyday lives, even in infancy, can be thought of as running experiments, evaluating results, and learning about the world around us. It's worth thinking about.
I've blogged this by request. Following my own dictates on playing expert, I should say that I'm not a philosopher. I've taken courses in logic, the philosophy of science, and even the philosophy of space and time, though, and I regularly talk to philosophers in a professional capacity. I hope I've handled the basics well enough to convey the flavor of a few issues in the area. If I've made any mistakes or oversights, please feel free to correct me or make additions in comments.