EVALUATING THE TRUTH OF REASONS AND ASSUMPTIONS

Common knowledge
It is obvious that no-one will be in a position to know whether all the reasons presented in
all the arguments that they may encounter are true. However, we all have a share in a body
of common knowledge, many of us have detailed knowledge about our particular field of
work or study, and we have some ideas about whom to trust to give us correct information
on subjects which are less familiar to us.
Common knowledge can take us a long way in assessing many of the short arguments we
looked at earlier. For example, we noted (p.25) that in the following argument, it was easy
for us to assess the first of the reasons:
One-third of the population still smokes. Everyone must know that smoking causes
lung cancer and heart disease. So, knowing the dangers of smoking is not sufficient
to stop people from smoking.
We may not know the accuracy of the claim that one-third of the population still smokes.
But we know that quite a number of people still smoke, because we see them doing so; and
the argument only needs to establish that some people still smoke, despite knowing the
dangers. The second reason – that everyone must know the effects of smoking – is more
difficult to assess. We observed that it depends upon an assumption that the publicity
about the dangers of smoking has been absorbed by everyone.
Perhaps one way to find out if this is so would be to interview smokers in order to discover
whether they believe that smoking is dangerous to health. If we found that many smokers
do not believe this, we would have produced a piece of additional evidence which would
cast doubt on the conclusion. (We shall say more about evaluating additional evidence in a
later section of this chapter.)
Reliability of authorities
We may sometimes need to assess the truth of statements by relying on other people as
authorities, perhaps because being certain about the truth of a particular statement
depends upon direct experience, which we lack. For example, we may find ourselves as
members of a jury having to assess the evidence of eye witnesses to a crime. We do not have
the direct experience of what happened, and we may hear two witnesses describing the
events in two conflicting ways. Another case in which we may have to rely on authorities is
where knowledge depends upon expertise, which we ourselves lack. We may, for example,
have to rely on the authority of scientists, because we lack the expertise to carry out for
ourselves the experiments which they claim establish the truth of something. Although we
cannot guarantee that by relying on the authority of others, we will never be mistaken
about anything, there are certain criteria we can use in order to minimise the chances of
being misled by other people. These criteria will be presented and applied in Chapter 4,
‘Evaluating evidence and authorities’.