Verificación de las Hipótesis Específicas

In our discussion of deductive arguments, we saw that to guarantee its conclusion, a deductive argument had to be valid and have true premises. This applied only to deductive arguments, but a more general criterion may be given for the success of any argument: For an argument to be successful -- to rationally convince its audience of its conclusion -- it needs to have strong inferences and acceptable premises.

We will not always be in a position to know whether every premise of an argument is true, but in this lecture we will consider ways of determining whether they are likely to be acceptable.

In any complex argument, some of the premises will be supported by subarguments, and some will not.

Clearly, not every premise can be supported by a further premise, or the arguments would have to go on forever. There are some things which will just have to be asserted.

What we need to think about in evaluating the premises of an argument is when the premises can just be accepted, and when further evidence would be required.

The main questions to consider in this regard when analysing an argument are:

1) Which premises can be accepted without further justification?

2) Of those which cannot, what sort of evidence would we need? Is such evidence provided?

Remember that which premises are considered acceptable is important, because no matter how strong the inferences in an argument are, if you don't accept the premises, you have no reason to accept the

conclusion.

7.3.1 Common knowledge

Whatever the context in which an argument is presented, there will be some common background knowledge which the audience of the argument will be expected to share. A proposition which can be assumed to be common knowledge in a particular context, can reasonably be offered as a premise without further support.

What counts as common knowledge will vary depending on the audience. The following propositions are all such as might be assumed to be common knowledge among Macquarie University students:

Macquarie University is in Sydney.

Trees are plants.

John Howard is the Prime Minister of Australia.

Some people are tall.

Unleaded petrol is better for the environment than leaded petrol.

If you don't breathe, you will die.

J comes before K in the alphabet.

Some of these are trivial, some aren't, but their common feature is that all of them are the kinds of propositions which could be used as premises in an argument (for the audience currently under consideration) without further justification.

If a premise is not common knowledge, we may require more evidence before we accept it.

Suppose someone wants to convince you that some animals were telepathic, and offers the following argument:

(3a) 1 Dogs are animals 2 Some dogs have telepathy therefore

C Some animals have telepathy

How strong is this argument? It is valid, so if you accept the premises, you will also have to accept the conclusion, but would you accept the premises? The situation with respect to each of these two premises is rather different. The first premise is the sort of claim which someone presenting an argument could

reasonably expect an audience to accept. That dogs are animals could be assumed to be common

background knowledge, and in ordinary circumstances it would be unreasonable to criticise the argument for failing to give any evidence for the first premise. The second premise, however, is certainly not

commonly known, and this is a premise which would need to be given some support in an argument.

7.3.2 The importance of common knowledge in constructing a broad standardisation

If a premise is not common knowledge, we need to have some reason to believe it. This is not a reason for undue scepticism, but it is a reason to think about why we believe the things we do. There is no point is simply disputing every premise of every argument which is not independently supported, but bear in mind when you go on to evaluate arguments that even for good arguments, the conclusion is only as certain as its premises.

(3b) 1 The intended purpose of airbags is to reduce the likelihood and severity of injuries in road accidents 2 they have been found to be ineffective in achieving this.

C: it is not worth paying extra to have airbags fitted when you buy a car.

This argument has two premises. The first might be acceptable as common knowledge, as anyone who knows enough about the matter to know what an airbag is would also be likely to know that this is their purpose. The second premise, however, which claims that airbags are not effective in reducing the likelihood and severity of injuries in road accidents, is something for which we would probably want extra evidence.

The fact that such evidence is not given is not a reason to reject the argument, but an appropriate attitude to take might be to admit that if it is true that they have been found to be ineffective, then it would not be worth paying extra to have airbags fitted. If you are not certain of the premise, then that uncertainty will carry across to the conclusion.

We will in the next lecture at when it is reasonable to accept some premise because of its source, but for the moment our concern is to think about how much of the argument needs to be included in a brief

standardisation, or considered in your evaluation. If there is a premise which is common knowledge, you may not need to include all premises which are given in support of it in your discussion, since even if the evidence given is not strong, the premise will still be accepted. But if premises are given in support of some proposition for which evidence is required, then it would be contrary to the principle of charity to omit such evidence. If you are planning to evaluate the argument, you need to evaluate a strong form of it, so it is unreasonable to leave out evidence which has been provided for premises which require such evidence.

Consider the longer passage, in which the argument above occurs.

(3b-i) As car buyers, it is often difficult to know what options are worth paying for and which are not.

Sometimes, we know that we are making decisions which are purely a matter of personal preference -- Do I want airconditioning? Do I prefer metallic paint? Can I live without a cup holder, or a CD player, or electric mirrors?

When it comes to airbags, though, people think that they are making an important choice about safety, but is this really the case?

If a car is involved in a collision, a small explosion is set off in the airbag, causing it to inflate rapidly. The intended purpose of airbags is to reduce the likelihood and severity of injuries in road accidents -- if you look at any manufacturers' information about airbags, this is always what it says, and we know that the original inventors of the airbags invented them to reduce the number of road deaths each year.

But, despite these intentions, they have been found to be ineffective for this purpose. A study of 10,000 motor vehicle accidents in the US in 1990 showed that there was no significant difference between the number of deaths and hospitalisations incurred in cars without airbags, and cars with airbags.

It follows that it is not worth paying extra to have airbags fitted. You might just as well spend the money on those cup holders.

The main argument is the one given above. How much else would we include if we were giving a standardisation for the purposes of evaluation? It was noted previously that "The intended purpose of airbags is to reduce the likelihood and severity of injuries in road accidents" is something which could be accepted as common knowledge, and would not be a subject of dispute in the evaluation of this argument.

The author of the argument does provide some evidence in support of this "if you look at any manufacturers' information about airbags, this is always what it says, and we know that the original

inventors of the airbags invented them to reduce the number of road deaths each year." But because noone is likely to object to this premise, it would be reasonable not to incorporate these extra premises in your broad standardisation, or to consider them explicitly in your evaluation.

What is more important is the evidence which is given for the more controversial claim, that airbags are ineffective. The evidence for this is the report of statistical research which had been conducted into the effectiveness of airbags. In evaluating the argument, it would be important to include this evidence. If you leave out the support an arguer has given for a controversial claim, you are not being fair to the author. You cannot object to a controversial premise without having considered the evidence which was given in support of it.

The principle of charity requires that you present the argument according to its best reasonable

interpretation. If you misrepresent the argument you are evaluating, and make it appear weaker than it is by leaving out evidence for important and controversial claims, you are committing the straw person fallacy.

Your criticisms will be unsuccessful, since they may not be relevant to the argument you are supposed to be evaluating .(More on this fallacy later).