The Language of Argument

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A p p l y i n g T h e s e M e t h o d s t o fi n d C a u s e s

check for bacteria that cause pneumonia-like diseases, and they had found

none. For this reason, attention was directed to the possibility that some pre-

viously unknown organism had been responsible but had somehow escaped

detection.

It turned out that an undetected and previously unknown bacterium had

caused the illness, but it took more than four months to find this out. The

difficulties encountered in this effort show another important fact about the

reliance on a background assumption: Sometimes it turns out to be false.

To simplify, the standard way to test for the presence of bacteria is to try to

grow them in culture dishes—flat dishes containing nutrients that bacteria

can live on. If, after a reasonable number of tries, a colony of a particular

kind of bacterium does not appear, then it is concluded that the bacterium

is not present. As it turned out, the bacterium that caused Legionnaires’

disease would not grow in the cultures commonly used to detect the pres-

ence of bacteria. Thus, an important background assumption turned out to

be false.

After a great deal of work, a suspicious bacterium was detected using

a live-tissue culture rather than the standard synthetic culture. The task,

then, was to show that this particular bacterium in fact caused the disease.

Again to simplify, when people are infected by a particular organism, they

often develop antibodies that are specifically aimed at this organism. In the

case of Legionnaires’ disease, these antibodies were easier to detect than the

bacterium itself. They also remained in the patients’ bodies after the infec-

tion had run its course. We thus have another chance to apply the NCT: If

Legionnaires’ disease was caused by this particular bacterium, then when-

ever the disease was present, this antibody should be present as well. The

suspicious bacterium passed this test with flying colors and was named,

appropriately enough, Legionella pneumophila. Because the investigators had

worked so hard to test such a wide variety of candidates, they assumed that

the disease must have some cause among the candidates that they checked.

So, since only one candidate remained, they felt justified in reaching a pos-

itive conclusion that the bacterium was a necessary condition of Legion-

naires’ disease.

The story of the search for the cause of Legionnaires’ disease brings out

two important features of the use of inductive methods in the sciences. First,

it involves a complicated interplay between what is already established and

what is being tested. Confronted with a new problem, established princi-

ples can be used to suggest theoretically significant hypotheses to be tested.

The tests then eliminate some hypotheses and leave others. If, at the end of

the investigation, a survivor remains that fits in well with our previously

established principles, then the stock of established principles is increased.

The second thing that this example shows is that the inductive method is

fallible. Without the background of established principles, the application

of inductive principles like the NCT and the SCT would be undirected; yet

sometimes these established principles let us down, for they can turn out to

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