72 Unit 2 Critical thinking: the basics
arguments. Flaws occur when weak claims are
expected to provide support for strong claims.
Not surprisingly, strong claims need equally
strong, or stronger, claims to support them
adequately. ‘You should never walk on frozen
lakes’ is not just strong: it is indefensible. It
would need to be assumed that no freshwater
ice, however thick, could bear a person’s
weight – which is obviously unwarranted.
In the next example the story is a bit
different, and so is the conclusion.
[5] People cross this lake every year from
November through to March. The ice can
be anything up to a metre thick. People
drive cars across it. I’ve even seen
bonfires on the ice at New Year and folk
sitting round having a party. So there is
no risk of anyone ever falling through in
the middle of February.Activity
Assuming the reasons are true, is this
argument sound, or does it have a flaw?Commentary
This is a classic example of anecdotal evidence
being used carelessly. The reasons are
insufficient for the conclusion they are being
used to support, even if you add all four of the
reasons together. The fact that people have
done various things on the ice in the past,
and come to no harm, does not mean there is
never going to be a risk in the future. In fact,
if some scientists are right about global
warming, what has been observed about
frozen lakes up until now will not be very
reliable evidence in years to come. On many
lakes the ice in February may become thinner
and less safe – just like the reasoning in [5]!people falling through the ice last year is a
very good reason for thinking twice about
walking on it now, and it would be irrational
not to think twice about it, if you value your
safety and you believe the story. But compare
[3] with the following case, which uses
exactly the same evidence:
[4] Three people fell through the ice last
winter when they were walking across
the lake. You should never walk on
frozen lakes.Activity
Discuss the difference between [3] and [4].Commentary
[3] is a sound argument and [4] is not. [4] is
flawed, like [1] and [2], and in the same way:
its conclusion is too general to draw from one,
or even three, particular pieces of (anecdotal)
evidence. In the right conditions it is perfectly
safe to walk on frozen lakes, and people do it
regularly. What happened to the three
unfortunate people who fell through the ice
was no doubt caused by the conditions being
unsafe at that time. But it doesn’t mean, as [4]
concludes, that frozen lakes are never safe.Insufficient reason
Another way to say what is wrong with [1],
[2] and [4] is that in each argument the
reason is insufficient or inadequate – i.e. not
strong enough – to support the conclusion. In
all three cases the argument goes too far, or
claims too much. In [3], by contrast, the
conclusion is much more limited in what it
claims: it just suggests a bit of caution.
Here we see again why the distinction
between strong and weak claims (Chapter 2.2,
page 25) is so important in evaluating some