2 2 1S u f f i c i e n t C o n d i t i o n s a n d N e c e s s a r y C o n d i t i o n sconfirmed, are always defeasible. (Recall Captain Cook’s discovery of black
swans at the start of Chapter 8.) That is why our inductive inference to the
conclusion that D is a sufficient condition could be refuted by the new data in
Case 4.The Necessary Condition Test
The necessary condition test (NCT) is like the SCT, but it works in the re-
verse fashion. With the SCT, we eliminated a candidate F from being the
sufficient condition for G, if F was ever present when G was absent. With
the NCT, we eliminate a candidate F from being a necessary condition for
G if we can find a case where G is present, but F is not. This makes sense,
because if G can be present when F is not, then F cannot be necessary for
the occurrence of G. Thus, in applying the necessary condition test, we only
have to examine cases in which the target feature, G, is present, and then
check to see whether any of the candidate features are absent.
NCT: Any candidate that is absent when G is present is eliminated as a
possible necessary condition of G.
The following table gives an example of an application of this test:
Table 2
Case 1: A B C D ~G
Case 2: ~A B C D G
Case 3: A ~B C ~D G
Because Case 1 does not provide an instance where G is present, it cannot
eliminate any candidate as a necessary condition of G. Case 2 eliminates A
as a necessary condition of G, since it shows that G can be present without
A being present. Case 3 then eliminates both B and D, leaving C as the only
possible candidate for being a necessary condition for G.
From this, of course, it does not follow that C is a necessary condition for
G, for, as always, new cases might eliminate it as well. The situation is the
same as with the SCT. An argument for a negative conclusion that a candi-
date is not a necessary condition, because that candidate fails the NCT, is a
deductive argument that cannot be overturned by any further cases. In con-
trast, an argument for a positive conclusion that a candidate is a necessary
condition, because that candidate passes the NCT, is an inductive argument
that can be overturned by a further case where this candidate fails the NCT.
For example, suppose we find:
Case 4: ~A ~B ~C ~D G
The information in this new Case 4 is enough to show that C cannot be a neces-
sary condition of the target feature G, regardless of what we found in Cases 1–3.
In applying both the SCT and the NCT, it is crucial to specify the tar-
get feature. Case 4 shows that candidate C is not a necessary condition for
target feature G. Nonetheless, candidate C still might be necessary for the97364_ch10_ptg01_215-238.indd 221 15/11/13 10:48 AM
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