One important aspect of reasoning is the ability to go further than the information you
have been given, to draw conclusions from evidence, to see what follows from statements
which other people make. This is an ability which we all exercise to a certain extent in our
daily lives. If we draw back the curtains in the morning, and find that last night’s snow
covering has gone, we conclude that the temperature must have risen overnight. If we
know that a friend has completed a 150 mile car journey in two hours, we conclude that
they must have exceeded the 70 mph speed limit.
Sometimes our conclusions will be more tentative than in these two examples. If we know
that a colleague’s children have all had bad colds recently, and we hear that colleague
sneezing throughout the day, then it is reasonable to conclude that they have caught a cold.
But they may not be suffering from a cold. Perhaps their sneezing is caused by an allergy to
something in the office, for example, a new pot plant or a new type of printing ink. In cases
like this, where the evidence points to a conclusion which may need to be reconsidered
in the light of further evidence, it is best to express our conclusion as something which is
‘probable’ or ‘likely’.
To improve our capacity for critical reasoning, we need to exercise the ability to draw
conclusions in a systematic way whenever we are presented with information – in discussions
with others, when reading newspapers and textbooks, when listening to the
comments of politicians. We may find it easiest to draw conclusions about those subjects
with which we are most familiar, but with practice, we can make progress in improving
the ability in relation to less familiar topics.
Let us turn to some examples to illustrate this. Consider the following passage:
Men with low blood cholesterol levels are more likely to develop intestinal cancer
than those with high blood cholesterol levels. But men who have high blood
cholesterol levels have an above-average risk of suffering a heart attack.
What conclusions can be drawn from this information? Can we conclude that it would be
a good thing for all men to aim to have a low blood cholesterol level, on the grounds that
this would reduce their risk of suffering a heart attack? No, we cannot conclude this from
the information available, because if they achieved a low blood cholesterol level they
would be more likely to develop intestinal cancer. So the most which can be concluded is
that lowering a patient’s blood cholesterol level in order to reduce the risk of heart attack
may increase the patient’s risk of intestinal cancer, and thus that it may not be wise to
attempt to lower a patient’s blood cholesterol level.
Note the tentative nature of this conclusion. It is possible that further information may
lead us to revise the conclusion. Suppose that intestinal cancer is a disease which usually
occurs in old age. In that case, lowering someone’s blood cholesterol level may move them
out of the group likely to die relatively young from a heart attack, and into the group likely
to live much longer, but also at risk of – eventually – developing intestinal cancer. In that
case, it may be wise to attempt to lower the blood cholesterol levels of those likely to suffer
heart attacks.
Let us look at another example:
Repeated spraying with the insecticide did not rid the tobacco fields of the insect.
Only the strongest of the species survived each spraying. When they mated, they
produced offspring more resistant to the insecticide than they were.
(Law School Admission Test, June 1983)
What can be concluded from this information? We know that the insects which were
strong enough to survive repeated spraying with insecticide produced offspring with even
greater resistance to the insecticide. In the original population of insects, there were
obviously some with weak resistance and some with strong resistance, so perhaps it is just a
matter of chance whether a particular insect has strong or weak resistance, and therefore
just a matter of chance that the offspring of the survivors had strong resistance.
But if it were just a matter of chance, then we should expect the new generation to include
some insects with weak resistance to the insecticide. The fact that they all had strong
resistance suggests that there is something about being the offspring of those with strong
resistance which makes insects more likely to have strong resistance. And this suggests that
resistance to insecticide, in at least some species, can be passed from one generation of
insects to the next. This is a useful conclusion to draw because it tells us that repeated
spraying with insecticide may not have the effect of eventually eliminating insect pests. It
may even have the effect of making the insect population stronger, if those which have
the resistance to the insecticide are strong in other respects as well, for example, in their
abilities to reproduce or to withstand adverse weather conditions and disease.
Here are some exercises for you to practise your skill in drawing conclusions.
For each of the following, say what conclusion you can draw from the passage:
1 The pond is frozen this morning. It was not frozen yesterday.
2 There is a flu epidemic sweeping through the school. Gitta, one of the pupils, has a
very high temperature and aching muscles, both of which are symptoms of flu.
3 The winter has been very severe. When we have a severe winter, the daffodils usually
come into flower late.
4 Jane arrived before Jim, although they set off at the same time, and they were both
travelling by car.
5 The murder victim died at 9 p.m. on Saturday. It is suspected that he may have been
poisoned, but it is not yet known whether it was poison or the blow to his head which
killed him. The injury to the head would have caused death instantly, had he still
been alive when he was hit. It has now been discovered that Ms Brown, the chief
suspect, was with friends 5 miles away from the murder scene between 7 p.m. and
10 p.m. on Saturday.
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