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

other animals, along with the rest of biology and science. It is not clear how

this doctor could go on practicing medicine. Moreover, there is usually no

practical alternative in real life. When faced with time pressure and limited

information, we have no way to judge new ideas without taking some back-

ground assumptions for granted.

A Detailed Example

To get a clearer idea of the complex interplay between our tests and the

reliance on background information, it will be helpful to look in some detail

at actual applications of these tests. For this purpose, we will examine an

attempt to find the cause of a particular phenomenon, an outbreak of what

came to be known as Legionnaires’ disease. The example not only shows how

causal reasoning relies on background assumptions, it has another interest-

ing feature as well: In the process of discovering the cause of Legionnaires’

disease, the investigators were forced to abandon what was previously taken

to be a well-established causal generalization. In fact, until it was discarded,

this false background principle gave them no end of trouble.

The story began at an otherwise boring convention:

The 58th convention of the American Legion’s Pennsylvania Department was

held at the Bellevue-Stratford Hotel in Philadelphia from July 21 through 24,

1976.... Between July 22 and August 3, 149 of the conventioneers developed
what appeared to be the same puzzling illness, characterized by fever,
coughing and pneumonia. This, however, was an unusual, explosive outbreak
of pneumonia with no apparent cause.... Legionnaires’ disease, as the illness
was quickly named by the press, was to prove a formidable challenge to
epidemiologists and laboratory investigators alike.^2
Notice that at this stage the researchers begin with the assumption that they
are dealing with a single illness and not a collection of similar but different
illnesses. That assumption could turn out to be wrong; but, if the symptoms
of the various patients are sufficiently similar, this is a natural starting as-
sumption. Another reasonable starting assumption is that this illness had
a single causative agent. This assumption, too, could turn out to be false,
though it did not. The assumption that they were dealing with a single
disease with a single cause was at least a good simplifying assumption, one
to be held onto until there was good reason to give it up. In any case, we
now have a clear specification of our target feature, G: the occurrence of a
carefully described illness that came to be known as Legionnaires’ disease.
The situation concerning it was puzzling because people had contracted
a disease with symptoms much like those of pneumonia, yet they had not
tested positive for any of the known agents that cause such diseases.
The narrative continues as follows:
The initial step in the investigation of any epidemic is to determine the character
of the illness, who has become ill and just where and when. The next step is to

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