An undercutting argument always strictly defeats the argument it
attacks, since (contrary to what happens in rebutting) it is not
counterattacked by the latter argument. In fact, the undercutter says
that the undercut argument does not work in the case at hand, while the
undercut argument does not say anything about its undercutter. Note that
the undercutter could also be viewed as an attack against the particular
instance of the default that is applied in the inference. For instance,
it may be said that undercutter in Figure 6 denies that the conditional
“if Fido is a pet dog, then presumably he is not aggressive” holds,
i.e., it denies that the fact that Fido is a pet dog is a reason for him
not to be aggressive (given the conditions in which Fido has been
raised). However, I prefer to view the undercutter as an attack against
a particular inference applying the general default, to stress that the
general conditional, stating a presumptive connection, is not affected
by the attack.
Let us consider an example pertaining to the epistemology of perceptions
(see Figure 7. Assume that in the park I see a bird that to me looks
pink (I perceive it in this way), and therefore I conclude that the bird
is pink. However, assume that I also see that the bird is a swan, which
leads me to conclude that the bird is white, as swans normally are.
However, since I know little about swans, I may remain in doubt about
the colour of the bird: am I seeing a special swan (are there any pink
swans around?) or is my perception of pink misleading me. Assume,
however, that I notice that there is a red sunset. Then, as I know that
even white things (not only pink ones) look pink under a red light, I
will conclude that the fact that the bird looks pink under these
conditions does not guarantee that it is indeed pink (it might as well
be white): this undercuts the inference from looking pink to
being so.