Analysing Reasoning

In this case, we do not have a conclusion indicator such as ‘so’ or ‘therefore’, but we do
have the word ‘cannot’. Is it being used to signal a conclusion? We must consider whether
the sentence in which it occurs is the main point which the passage is trying to establish.
It seems that the passage is trying to convince us that Falstaff cannot have dieted, and we
seem to have a clear argument if we rearrange it to read:
People who diet lose weight. Falstaff hasn’t lost weight. Therefore, he cannot have
dieted.
This is the most natural way to read the passage.
But suppose we had started out by assuming that the main point which the passage was
aiming to get us to accept was that Falstaff has not lost weight. Then, we would have set
out the argument as follows:
People who diet lose weight. Falstaff cannot have dieted. Therefore, he hasn’t lost
weight.
But this is an unnatural reading of the passage, in two respects. First, it would not be
natural to use the words ‘cannot have dieted’ in the second sentence if the meaning it
aimed to convey was that Falstaff has been unable to diet. Second, even if we replaced
‘cannot have dieted’ with ‘has been unable to diet’, the first two sentences would be
insufficient to establish the conclusion, since Falstaff may have lost weight by some means
other than dieting, for example by taking exercise. Moreover, the kind of evidence which
one would have to use in order to establish that Falstaff had not lost weight would be
evidence, not about whether or not he had dieted, but about what he weighed in the past
compared with what he weighs now.
Here is another example in which there are no conclusion indicators such as ‘so’ and
‘therefore’:
We need to make rail travel more attractive to travellers. There are so many cars on
the roads that the environment and human safety are under threat. Rail travel
should be made cheaper. Everyone wants the roads to be less crowded, but they still
want the convenience of being able to travel by road themselves. People will not
abandon the car in favour of the train without some new incentive.
What is the main point which this piece of reasoning tries to get us to accept? Clearly it is
concerned with suggesting a way of getting people to switch from using cars to using
trains, on the grounds that it would be a good thing if people did make this switch. We
could summarise the passage as follows:
Because the large numbers of cars on the roads are bad for the environment and
human safety, and because people will not abandon the car in favour of the train
without some new incentive, we need to make rail travel more attractive. So, rail
travel should be made cheaper.
Notice that the word ‘should’ appears in the conclusion. This may have helped you to see
which sentence was the conclusion. Now that we can see more clearly what the argument
is, we may question whether it is a good argument. For example, is it the cost of rail travel
which deters motorists from switching to using trains, or is it because rail travel is less
convenient? Would reducing rail fares really make a difference? Are there any alternative
measures which would better achieve the desired effect? Setting out the argument in
this way can help us to see what questions we need to ask when we begin to evaluate
arguments.