The Language of Argument

2 3 4

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

Concomitant Variation

The use of the sufficient condition test and the necessary condition test de-

pends on certain features of the world being sometimes present and some-

times absent. Some features of the world, however, are always present to

some degree. Because they are always present, the NCT will never eliminate

them as possible necessary conditions of any event, and the SCT will never

eliminate anything as a sufficient condition for them. Yet, the extent or degree

to which a feature exists in the world is often a significant phenomenon that

demands causal explanation.

An example should make this clear. In recent decades, a controversy has

raged over the impact of acid rain on the environment of the northeastern

United States and Canada. Part of the controversy involves the proper in-

terpretation of the data that have been collected. The controversy has arisen

for the following reason: The atmosphere always contains a certain amount

of acid, much of it from natural sources. It is also known that an excess of

acid in the environment can have severe effects on both plants and animals.

Lakes are particularly vulnerable to the effects of acid rain. Finally, it is also

acknowledged that industries, mostly in the Midwest, discharge large quan-

tities of sulfur dioxide (SO 2 ) into the air, and this increases the acidity of

water in the atmosphere. The question—and here the controversy begins—is

whether the contribution of acid from these industries is the cause of the en-

vironmental damage downwind of them.

How can we settle such a dispute? The two rules we have introduced

provide no immediate help, for, as we have seen, they provide a rigorous

test of a causal hypothesis only when we can find contrasting cases with

the presence or the absence of a given feature. The NCT provides a rigorous

be false. The discovery of the false background principle that hindered the

search for the cause of Legionnaires’ disease led to important revisions in

laboratory techniques. The discovery that certain fundamental background

principles are false can lead to revolutionary changes in science.

1. Sometimes we describe necessary conditions as causes, but at other times
   we describe sufficient conditions as causes. Why? Be sure to give at least
   two different examples of each pattern.


2. Legionella pneumophila is necessary for Legionnaire’s disease, but so are
   being alive, having blood, and so on. Why do we think that Legionnaire’s
   disease is caused by Legionella pneumophila instead of being caused by being
   alive, having blood, and other necessary conditions?

discussion Questions

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