At some point, a fact becomes relevant

 Here’s a little more on thinking and how we form hypotheses when we are on the trail of a problem.

The philosopher Carl G. Hempel tells the classic story of Dr. Ignaz Semmelweiz, who solved the problem of why so many women died at the First Maternity Division at Vienna General Hospital in the 1840s.

From 1844 to 1846, 7 to 11 percent of the women who came to give birth in the First Division died of childbed fever.

In the Second Division, the rates were 2 to 3 percent.

The answer, of course, was that the hospital was a prestigious institution where medicine was taught. The faculty and students were using instruments in autopsies and then using them in the maternity ward with only superficial cleaning.

The problem is obvious now, and medical practitioners now take care in sterilizing their instruments to prevent infections. But the problem wasn’t obvious then. Semmelweiz solved the problem by testing several hypotheses.

One was that childbed fever was spread by crowding. He checked, and found that the Second Maternity Division actually was more crowded, in part because women were trying desperately to avoid the notorious First Division.

Psychiatry was in its early stages then, and some of Semmelweiz’s colleagues thought that the presence of a priest, going through the wards with a procession of assistants to administer last rites to dying women, might have a distressing and weakening affect on the others. Semmelweiz arranged to sneak the priest through the wards on his rounds. 

The story goes on, but you can see why a scientific problem is not just about getting the facts.

What counts as a relevant fact changes as the hypothesis changes. If you think that crowding might explain childbed fever, the relevant facts involve the numbers of patients on the wards. If you think that stress caused by priestly processions might explain childbed fever, the relevant facts involve the priest and his passage through the wards.

It turned out that the relevant facts involved microbes. But no one suspected that was possible when Semmelweiz set out on the trail. Those facts became relevant when he formed a hypothesis that microbes were somehow causing childbed fever in a way that was not known to medical science at the time.

A fact becomes relevant when it's attached to a hypothesis, that is, to a possible explanation.

• Source: Carl G. Hempel, Philosophy of Natural Science; Englewood Cliffs, N.J.,  Prentice-Hall, Inc., 1966.