The Language of Argument

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C H A P T E R 1 0 ■ C a u s a l R e a s o n i n g

Bellevue-Stratford and being susceptible to the disease could not be ruled

out by the SCT as a sufficient condition of getting the disease.

As soon as spending time at the Bellevue-Stratford became the focus of

attention, other hypotheses naturally suggested themselves. Food poison-

ing was a reasonable suggestion, since it is part of medical knowledge that

diseases are sometimes spread by food. It was put on the list of possible

candidates, but failed. Investigators checked each local restaurant and each

function where food and drink were served. Some of the people who ate in

each place did not get Legionnaires’ disease, so the food at these locations

was eliminated by the SCT as a sufficient condition of Legionnaires’ dis-

ease. These candidates were also eliminated by the NCT as necessary con-

ditions because some people who did get Legionnaires’ disease did not eat

at each of these restaurants and functions. Thus, the food and drink could

not be the cause.

Further investigation turned up another important clue to the cause of

the illness.

Certain observations suggested that the disease might have been spread through

the air. Legionnaires who became ill had spent on the average about 60 percent

more time in the lobby of the Bellevue-Stratford than those who remained well;

the sick Legionnaires had also spent more time on the sidewalk in front of the

hotel than their unaffected fellow conventioneers.... It appeared, therefore, that

the most likely mode of transmission was airborne.^5

Merely breathing air in the lobby of the Bellevue-Stratford Hotel still could

not be a necessary or sufficient condition, but the investigators reasoned

that something in the lobby air probably caused Legionnaires’ disease, since

the rate of the disease varied up or down in proportion to the time spent in

the lobby (or near it on the sidewalk in front). This is an application of the

method of concomitant variation, which will be discussed soon.

Now that the focus was on the lobby air, the next step was to pinpoint a

specific cause in that air. Again appealing to background medical knowl-

edge, there seemed to be three main candidates for the airborne agents that

could have caused the illness: “heavy metals, toxic organic substances, and

infectious organisms.” Examination of tissues taken from patients who had

died from the disease revealed “no unusual levels of metallic or toxic organic

substances that might be related to the epidemic,” so this left an infectious

organism as the remaining candidate. Once more we have an application

of NCT. If the disease had been caused by heavy metals or toxic organic

substances, then there would have been unusually high levels of these sub-

stances in the tissues of those who had contracted the disease. Because this

was not always so, these candidates were eliminated as necessary conditions

of the disease.

Appealing to background knowledge once more, it seemed that a bacte-

rium would be the most likely source of an airborne disease with the symp-

toms of Legionnaires’ disease. But researchers had already made a routine

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