2.8 Reasons 59
Its orbiting of the Sun justifies the claim that
Mars is a planet. If I did not already know that
Mars was a planet, [4] would give me a reason
to believe it (provided I knew that planets are
objects that revolve around suns).
To summarise so far, there are two ways in
which a claim can be understood as a reason:
as grounds for drawing a conclusion, or as an
explanation. Usually you can tell from the
meanings of sentences what their functions
are, or from the context surrounding them.
Sometimes, however, it is quite difficult to tell,
especially if a short passage is taken out of
context. But there is another complication,
too: sometimes argument and explanation are
both recognisable in a text at the same time.
Indeed sometimes an argument consists of an
explanation. Some or all of these complications
are reflected in the following examples.Discuss the following pairs of sentences.
Can either of the sentences in each case be
understood as a reason for the other? If so,
what kind of reason?
[5] Tax rises are not vote-winners. In
the last four decades, every time a
government has raised taxes, their
poll-ratings have fallen significantly.
[6] The government will not raise taxes
this close to a general election. The
result could be very close and tax
rises are not vote-winners.
[7] The accused was at her desk in the
office at 3 p.m. but no one reported
seeing her again until after 4. That
was plenty of time to get to the
scene of the crime and back.Activity
Commentary
We’ll take these examples in turn, starting
with [5]. This is not exactly a trick question,
but it is a tricky one. The straight answer is
that either of the sentences could beIn logic the term ‘premises’ is preferred over
‘reasons’. In critical thinking it tends to be the
other way around, although there are
exceptions. This is because critical thinking is
a less formal subject than logic. In this book
we have used both words, and up until now
treated them as having roughly the same
meaning when used in connection with
arguments. However, there are differences
which sometimes make one term more
appropriate to use than the other. ‘Premise’,
being the more formal word, is defined by its
position in an argument – literally meaning
‘placed before’ – whereas a reason is
identifiable more by its meaning: what it
claims. Logicians often work with symbols
rather than sentences. In an argument such as:
P & Q therefore R‘P’ and ‘Q’ are premises. But nothing about these
letters makes them recognisable as reasons for
‘R’. You would have to know what ‘P’ and ‘Q’
stand for – and ‘R’ too – before you could
recognise them as grounds for believing R.
Relevance
For one thing, a premise cannot be understood
as a reason for a conclusion unless it is relevant
to the conclusion. Suppose someone tried to
argue that:
[3] Seawater is salty, so Mars is a planet!The premise of this ‘argument’ is true, and so
is the conclusion. But knowing that seawater
is salty gives no reason to believe that Mars is
a planet, since the two claims are completely
unrelated. In [3] the second claim is known as
a ‘non sequitur’, because it does not follow
from the premise in any logical sense of the
word, even though both claims are true. Nor,
for that matter, does the saltiness of water
explain why Mars is a planet.
Compare with the following argument:
[4] Mars is a planet since it can be seen to
orbit the Sun.