These are arguments that are set up in such a way that nothing could possibly refute
them; thus, they seal themselves off from criticism. We can bring out just why they are
objectionable in a couple of ways:
Such an argument is objectionable because it provides no one who is skeptical with any
reason to believe it. Usually, a skeptic would want a theory (T) to prove itself -- to
show that it gets the right result in a situation where a competing theory (T') does not.
Note that here we have what we can call a crucial test: if the test comes out one
way, it favors T and not T'; if it comes out in the another way, it favors T' and not T.
The problem with self-sealing theories (or arguments) is that there is no such test for
them: any apparently problematic data can be accommodated within them Thus, there is no
reason given the skeptic for believing in them.
Another way to think of this is by analogy with tautologies, or sentences
which are true in every possible situation. A self-sealing theory will be true in every
possible circumstance -- any attempt to falsify it fails. Thus, it is similar to a
tautology, such as "I am here or I am not here." Sentences such as this one are
also true in every possible circumstance, but it is for this reason that they tell us
nothing that we didn't already know. They give us no information, and so can be said to
have no content. By analogy, a self-sealing theory (argument) tells us nothing about the
world either -- more specifically, it tells us nothing about the specific domain to which
it applies. Therefore, it is vacuous.
One might think that a theory could look in practice like a self-sealer if it is the true
empirical theory of some subject matter. There is an important difference between such a
theory and a self-sealer: if the theory is not self-sealing, then one can infer testable
hypotheses such that if the relevant evidence were to come out in one way, these tests
would count against the theory. In fact, it is through its performance in such tests that
such a theory would likely establish itself as the true theory. By contrast, a
self-sealing theory doesn't even imply such tests.
An argument can be self-sealing in a number of different ways:
Dangers of Self-Sealing Arguments: