Thinking and Deciding

HYPOTHESES IN SCIENCE 163
to administer the last rites to a dying patient. The hypothesis was that this demoralized
the other patients, already weakened by childbirth, and caused their deaths.
Semmelweis induced the priest to enter the wing by a roundabout route without his
bell, so that he would not be noticed. Again, there was no change. New mothers still
died.
A colleague of Semmelweis’s died of an infection received from a scalpel that
had been used to perform an autopsy. Semmelweis noticed that the colleague’s symptoms
resembled those of puerperal fever. It occurred to him that deliveries in the First
Division were done by medical students and physicians who often had just performed
autopsies, whereas deliveries in the Second Division were done by midwives. He hypothesized
that “cadaverous matter” — material from the corpses that stuck to the
students’ hands — might be the cause of the disease. To test this hypothesis, he
induced the students and physicians to wash their hands in a solution of chlorinated
lime, which he thought would remove the cadaverous matter, before delivering babies.
This succeeded. The incidence of puerperal fever was sharply reduced, to the
level of the Second Division.
Following this great success, on one occasion Semmelweis and his colleagues
examined a woman in labor who suffered from a festering cervical cancer and then
proceeded to examine twelve other women without washing their hands, confident
that they could do no harm, because the first woman was alive. Eleven of the twelve
others died of puerperal fever. This tragedy convinced Semmelweis to broaden his
hypothesis to include “putrid living matter” as well as cadaverous matter.
Of course, Semmelweis was still not quite right. As we know today (after the
laborious testing of many other hypotheses by many scientists), bacteria were the
problem. Students and interns transferred the bacteria from the corpses of women
who had died of the puerperal fever to women who were not yet infected. Semmelweis
did not end the disease, because there were other paths by which the bacteria
could be transmitted. Of course, Semmelweis’s work was by no means in vain. It
led others, such as Pasteur, more directly to the germ theory, and it saved many
lives along the way. Incorrect hypotheses may still be of considerable value, both in
practical terms and in narrowing the search for further hypotheses.
Semmelweis’s work illustrates nicely how hypothesis testing often works. To
use the terms of the search-inference framework, a hypothesis is a possibility. The
hypothesis usually concerns a possible cause of some event. Roughly, a “cause” is
an event or state whose presence or absence would make the event in question occur
or not occur, if the hypothesized cause could be manipulated. 2
In the simplest sort of hypothesis testing, we imagine some result that would
definitely be obtained if the hypothesis were true, and we look for that result. For example,
Semmelweis’s hypothesis that the position of delivery was important implied
that changing the position would reduce the incidence of the disease. This result was
2 Often, causes cannot be manipulated. When we say that the shape of the moon’s orbit is “caused,” or
determined, by the earth’s gravity, we cannot test this by removing the gravity and seeing what happens.
Still, to say that the orbit is determined by gravity is to make a claim about what would happen if we could
remove the gravity. Note also that the term cause, as used here, includes partial causes, or influences.