64 Unit 2 Critical thinking: the basics
Is this a sound argument? As it stands, no: it is
incomplete. Without an additional premise, to
the effect that genuine banknotes have unique
numbers, the conclusion does not follow from
the single, stated premise. (If all, or even some,
genuine banknotes had duplicate numbers,
then obviously the first clause of [2] would not
be a good reason for claiming that the notes
weren’t genuine.) In [2], therefore, it is
implicitly assumed that if the banknotes were
all genuine they would have different
numbers. And because it is necessary to
assume this for the argument to make sense,
we treat it like an unspoken premise.
In standard form:
[2a] R (stated)
These banknotes all have the same
serial number.
A (unstated)
Genuine banknotes all have unique
serial numbers.C These banknotes can’t all be
genuine.By contrast with the claims in [1], R and A are
both well justified: R by the photographicadvance ahead of detection; or, alternatively, if
the technology for detection does not catch up
in the foreseeable future. The fact that forgery is
outstripping detection at present does not mean
the balance won’t change. By drawing the
conclusion that we are ‘going to see ever-
increasing amounts of counterfeit money’, the
author is assuming more than he or she is
saying. And because these assumptions about
the future are extremely questionable, [1] is not a
reliable argument.Hidden premises
Another way to think of implicit assumptions
is as missing, or hidden, premises. They are
premises because they are necessary for the
success and soundness of the argument. They
are hidden because they are unstated. In the
example above, there was at least one hidden
premise that was unwarranted, making the
argument as a whole unacceptable. But
implicit assumptions need not always be
detrimental to an argument.
Here is another case to consider, on the
same topic:[2] These banknotes all have the same
serial number, so they can’t be genuine.