254 Unit 7 Critical reasoning: Advanced Level
can apply to boats, buildings and so on, as
well as to arguments. Likewise, ‘valid’ and
‘invalid’ can be used to describe a whole range
of objects, material and abstract. A rail ticket
is valid for certain journeys but not for others,
and is invalid if it is out of date. An argument,
or form of reasoning, is invalid if its premise
could be true and its conclusion false.
Likewise it is valid if, whenever the premises
are true, the conclusion cannot be false.
It is crucial to note that when logicians talk
about validity they are talking about forms of
argument, not just about individual
arguments. An argument is valid or invalid by
virtue of its form. Individual arguments are
different from each other because they are
made up from different sentences with
different meanings; but countless different
arguments can share the same form. In fact, if
you think back to Chapter 2.5, you will
remember that all arguments have the same
basic or ‘standard’ form:[1] R 1 , R 2 ,... Rn / Cor
R 1
R 2... Rn
C
where ‘R’ stands for a reason or premise, and
‘C’ for a conclusion. The separator ‘/’, or the
horizontal line, stands for the logical relation
of ‘following from’, and is roughly equivalent
to the word ‘so’ or ‘therefore’.
Since [1] is the form of any argument
whatsoever, it is obviously not a valid form,A good argument is one that can be trusted. If
the reasons from which it starts are true, we need
to know that the conclusion will be true too, and
true for the reasons given. An argument which
gives that assurance, and whose reasons are
warranted, can be rated as sound. An argument
which fails on either of those counts is unsound.
Critical evaluation of an argument basically
means judging its soundness.Validity
Obviously, if we don’t accept the reasons
(premises) that are given for a conclusion then
we cannot trust the conclusion either. But
even if we do accept all the reasons as true, we
may still find, on inspection, that what is
inferred from those reasons simply does not
follow. Thirdly, there are many instances in
which we simply don’t know whether the
reasons are true or not, but we still want to
know that the reasoning is good, so that if the
premises are true we can be sure that the
conclusion would be true as well. An argument
that gives that assurance is said to be valid.
And it remains valid – though not sound –
even if the premises are known to be false.
What we need, therefore, is a way of judging
the quality of reasoning in an argument that is
independent of the truth of the premises; or at
least which sets aside the truth-or-falsity issue
whilst judging the quality of the reasoning in
isolation. The discipline which provides the
methodology for this judgement is logic.
Logic
We saw in Chapter 2.10 that the word ‘sound’
has both a special meaning in logic and
critical thinking, and a general meaning. So it