The second type of assumption is one which is needed to fill a gap within the argument,
either as an additional reason, without which the reasons which are offered do not fully
support the conclusion, or as a missing link between the reasons and the conclusion. Here
is an example of an argument which illustrates the former:
In tests designed to investigate the effect of a time delay on recalling a list of words,
subjects remembered fewer words after a 30-second delay than after a 10-second
delay. Therefore, after a 60-second delay, we would expect subjects to remember
even fewer words than after a 30-second delay.
Before going on, ask yourself what is being assumed. Write down any assumption you can
identify.
The argument gives just one reason for its conclusion that subjects can be expected to
remember fewer words after a 60-second delay than after a 30-second delay. The reason
is the piece of evidence that fewer words are remembered after 30 seconds than after
10 seconds. But this piece of evidence supports the conclusion only if it is true that the
ability to recall goes on declining after a 30-second delay. So the argument is relying on this
assumption in order to draw its conclusion. If we did not make this assumption explicit,
we might happily accept the conclusion as obviously following from the evidence. Even
when the assumption has been identified, we may consider it a reasonable assumption to
make. Nevertheless, it is possible that subjects would be able to remember just as many
words after 60 seconds as after 30 seconds, perhaps because the number of words still
retained in the memory was a manageable number for the memory to hold. Self-respecting
psychologists would not be prepared to draw a firm conclusion without carrying out an
appropriate further test.
Here is another example in which one of the reasons has been left unstated:
When cigarette advertising is banned, cigarette manufacturers save the money they
would otherwise have spent on advertising. Thus, in order to compete with each
other, they reduce the price of cigarettes. So, banning cigarette advertising leads to
an increase in smoking.
Before reading further, think about the reasoning in this passage. What conclusion is it
trying to get us to accept? What basic reason does it offer? Is there an intermediate
conclusion? Can you identify a stage in the argument which has not been stated?
The argument starts with a basic reason:
When cigarette advertising is banned, cigarette manufacturers save the money they
would otherwise have spent on advertising.
From this it draws the conclusion (an intermediate conclusion):
Thus, in order to compete with each other, they reduce the price of cigarettes.
It then draws the main conclusion:
So, banning cigarette advertising leads to an increase in smoking.
The main conclusion would not follow from the intermediate conclusion if a reduction in
the price of cigarettes made no difference to the numbers of cigarettes bought and smoked.
So an assumption underlies this move – that when cigarettes are cheaper, smokers smoke
more, or non-smokers become smokers. The conclusion does not say exactly what it means
by ‘an increase in smoking’, so we cannot be sure whether the assumption is:
When cigarettes are cheaper, smokers smoke more,
or
When cigarettes are cheaper, more people smoke,
or perhaps both of these. However, it clearly requires at least one of these assumptions
in order to support the conclusion, and perhaps both assumptions are questionable. This
is a case of an assumption which, taken together with an intermediate conclusion, gives
support to the main conclusion of the argument.
In some pieces of reasoning, an intermediate conclusion may be left unstated. Imagine the
following report being made by a policeman to his superior officer about a theft from an
art gallery.
The burglar must have left by the fire escape. This person is not in the building now,
but has not been seen leaving the building, and there are guards posted at each
entrance.
What intermediate conclusion is the policeman drawing which he has not actually stated?
Is this a reasonable conclusion to draw?
The policeman gives three reasons which, taken together, are intended to support the
conclusion that the burglar must have left by the fire escape:
Reason 1: This person is not in the building now
supports the claim that the burglar must have left the building. But
Reason 2: (the person) has not been seen leaving, and
Reason 3: there are guards posted at each entrance
do not entitle us to conclude that the burglar must have left by the fire escape unless we
assume that Reason 3 supports an intermediate conclusion to the effect that no one could
leave undetected except by the fire escape. This assumption, taken together with Reasons 1
and 2, give strong support to the conclusion. However, the assumption itself is open to
dispute. Perhaps the guards were insufficiently watchful, or failed to recognise the burglar
as a burglar, or perhaps it is possible for someone to leave the building undetected through
a window on the ground floor.
In the above examples, we have often found that identifying an assumption has led us to
question the truth of that assumption, and perhaps to reserve judgement on an argument
until we have obtained further evidence or information. But sometimes when we have
identified an assumption, we will see that there is no good reason to think it is true, and we
will therefore judge the argument to be unsound. Consider the following example:
Some people say that the depiction of violence on television has no effect on viewers’
behaviour. However, if what was shown on television did not affect behaviour,
television advertising would never influence viewers to buy certain products. But we
know that it does. So it cannot be true that television violence does not affect
behaviour.
See if you can pick out the missing assumption here, and say what is wrong with it.
At first sight, this looks like a plausible argument, and many people will be tempted to
accept that it is successful in establishing its conclusion. Yet, whichever way we interpret
it, it rests on a dubious assumption. One way of interpreting it is to see it as relying on
the assumption that, on the one hand, the depiction of violence on television and, on the
other hand, advertising on television are alike in important ways – indeed, in ways which
allow us to conclude that if one affects the behaviour of viewers, the other one must also
affect the behaviour of viewers. But the only thing which they have in common which is
mentioned in the argument is that both are shown on television.
Perhaps they are alike in some respects, for example, in that they are dramatic, and likely to
make an impact on viewers in such a way that viewers remember them. But perhaps the
differences between them make a difference to their effects on viewers’ behaviour. They
are different in that programmes depicting violence are not trying to sell violence, not
trying to make it attractive to the viewer. There may also be a difference in that most
people’s natural response to violence is not one of approval, whereas they may well approve
of and aspire to some of the lifestyles depicted in advertisements. So the assumption that
the two are alike in ways which are relevant to their possible effects on viewers’ behaviour
is questionable.
There are two other possible interpretations of the passage, each of which rests on a
dubious assumption. It may be suggesting that because television advertising affects
viewers’ behaviour, everything shown on television, including depictions of violence, must
affect behaviour. In that case, the dubious assumption is that if one aspect of television
output affects behaviour, all aspects must. Alternatively, it may be suggesting that the
example of advertising demonstrates that some things shown on television affect
behaviour. In that case, in drawing its conclusion, it relies on the wholly implausible
assumption that if some things which are shown on television affect behaviour, then
violence shown on television must be one of those things.
The discovery that this argument does not give strong support to its conclusion does not
establish that its conclusion is false. Perhaps violence shown on television does affect
viewers’ behaviour, but, if this is so, it is a truth which cannot be established by means of
this particular argument. The ability to identify the mistakes in other people’s reasoning
is a valuable skill which will be discussed in more detail in the next chapter.
The examples discussed above have been of specific assumptions relating to the subject
matter of particular arguments. There are some assumptions which form the whole
context in which an argument is presented, but which may not be made explicit, so
that someone unfamiliar with the context will find it more difficult to understand the
argument. Consider the following passage:
It has been claimed that powdered rhinoceros horn has aphrodisiac properties, but
scientists investigating its effects have been unable to find any chemical effect on the
human nervous system. Also, an experiment was carried out in which 100 people
ate powdered rhinoceros horn, and another 100 people ate powdered rice, without
knowing what they were eating. Very many more of those who ate the rice reported
feeling an increase in sexual arousal than did those who ate the rhinoceros horn.
This demonstrates that rhinoceros horn probably does not have aphrodisiac
properties.
In describing the experiment, and making the claim about what it demonstrates, this
argument does not bother to state that powdered rice is not an aphrodisiac. But we can
understand that this is being taken for granted, if we reason as follows:
If rhinoceros horn has aphrodisiac properties, then more people should report an
increase in sexual arousal after eating rhinoceros horn than after eating powdered
rice, which we know does not have aphrodisiac properties. But this did not happen
in the experiment. So rhinoceros horn does not have aphrodisiac properties.
Someone familiar with the way in which such experiments are carried out – the use of a
control group of people with which to compare those on whom the rhinoceros horn
is tested, the attempt to eliminate irrelevant psychological effects by keeping subjects
ignorant of which substance they are eating – will readily understand why the conclusion
is being drawn, and will see that there is an unstated assumption that powdered rice is not
an aphrodisiac.
Someone unfamiliar with the context of experiments may find it more difficult to
understand what is going on. They may, of course, notice that nothing is said about the
aphrodisiac properties of powdered rice, and they may reason as follows:
Powdered rice either does or does not have aphrodisiac properties. If it does,
then the experiment cannot tell us whether rhinoceros horn has no aphrodisiac
properties or merely weaker aphrodisiac properties than does powdered rice. If it
does not, then the experiment does indicate that rhinoceros horn does not have
aphrodisiac properties, because if it did have such properties, the number of those
reporting an increase in sexual arousal should have been higher amongst those who
ate rhinoceros horn than amongst those who ate powdered rice.
However, this a complex piece of reasoning, and, rather than hitting upon this, readers
of the argument might instead imagine a context in which it is not known by the
experimenters whether either substance has aphrodisiac properties. They might then conclude
that the experiment appeared to indicate that both substances have aphrodisiac
properties, although the powdered rice had much stronger aphrodisiac properties than the
rhinoceros horn. So they might regard the conclusion of the argument as mistaken, even
though, provided one assumes that powdered rice is not an aphrodisiac, it is a reasonable
conclusion to draw from the evidence.
This is an example, then, of an argument with a specific unstated assumption, which it will
be more difficult to identify if one is unfamiliar with the context – the whole set of
background assumptions – in which the argument is set. This indicates the value
of understanding certain contexts of arguments, and that it is valuable to ask certain
questions about any argument which cites experimental evidence – for example, what is
the purpose of any comparison which is being made between different groups of people,
what differing conclusions could be drawn on the basis of one set of assumptions as
opposed to a conflicting set of assumptions?
We have said little here about assumptions as to the meanings of words and phrases
used in reasoning, but we shall discuss this in greater detail in Chapter 5. The following
exercises will enable you to practise the skill of identifying assumptions.
Summary: Identifying assumptions in an argument
1 In Critical Thinking, an assumption of an argument is something that has not
been stated, but upon which the argument depends.
2 Within an argument, an assumption can function as a basic reason, as an
additional reason or as an intermediate conclusion.
3 Assumptions in an argument may or may not be true.
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