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and so on, they invite pop stars to their parties and comport themselves as if they too were pop stars, just out to sell themselves really.

In this example the speaker gets side-tracked into commenting upon the prime minister's suits and party guest-lists, and fails, beyond the vague charge that the government's policies are 'poll-driven nonsense', to offer a substantive criticism. Most of what is said is at best only obliquely relevant to the issue.

4 As with conclusions, there are certain words that are usually (but not always) reliable indicators of the presence of premises - premise indicators. We have already seen some of these because they mark the speaker's or writer's move from premises to conclusion or from conclusion to premises ('since', 'because', 'is implied by', and so on.) There are other words and phrases that introduce sentences stating a premise or premises. A speaker or writer might state their conclusion and then begin the next proposition with such phrases as:

For example:

I put it to you that Ms White killed Colonel Mustard in the ballroom with the candlestick. The reason I make this claim is that on the night of Colonel Mustard's death Lady Scarlet saw Ms White in the ballroom beating Colonel Mustard over the head with the very candlestick that was later found to have Ms White's fingerprints and Colonel Mustard's blood on it.

Other premise indicators may occur at the beginning of a sentence containing both the premise and the conclusion. For example:

On the basis of the fact that they have promised big tax cuts, I conclude that the Conservative Party will probably win the next general election.

We should be aware, however, that indicator words can be used with other meanings. The sentence, 'Since 2000 I have been a student at the University of Anytown' does not express the conclusion of an argument because 'since' is used here to talk about a period of time. 'Because' can be particularly tricky as it is frequently used in explanations that are not intended as attempts to persuade. These are cases where we have to think hard about the context of the word's use and what is the most likely intention of the speaker or writer. We need to work out whether they are telling us that such-and-such event occurred as a result of some other event - that is, whether they intend to assert a causal relationship. For in that case, 'because' is being used to introduce an explanation, not an argument. For example:

The tap is leaking because it needs a new washer.

This is an explanation of why the tap is leaking; 'because' is used to indicate a causal relationship rather than a connection between premise and conclusion.

This dual role of 'because' - its role in both arguments and explanations - can be confusing where the explanation of actions is concerned. Suppose you are driving fast and your passenger asks, 'Why are you driving so fast?' You assume your passenger is not in any way suggesting that you shouldn't drive so fast. You think they don't mind in the slightest. You assume they are merely curious as to why you're driving fast - whether it's because you're late, being chased by the police, or perhaps testing the limits of your new car. Your reply, however, is simply 'Because I enjoy it'. This would be an explanation: you are telling your passenger why you're driving fast, not trying to persuade them of some further proposition.

But suppose when your passenger asked 'Why are you driving so fast?', you didn't assume that they didn't mind your driving fast. You think perhaps they do mind. So you take the question as demanding a justification for your driving so fast. If you now say 'Because I enjoy it.', then you would be arguing, roughly, that it is all right to drive at such a speed on the grounds that you have a right to do what you like. In that case 'Because I enjoy it.' would be a premise of an argument, which might initially be expressed thus:

It's OK for me to drive as fast as I like, because I like driving fast. I think we should be free to do anything that we enjoy.

It might be re-written thus in standard form:

P1) I enjoy driving fast.

P2) It is acceptable for me to do anything I enjoy.

C) It is acceptable for me to drive fast.

As demonstrated by the following examples, 'Since', 'thus' 'so' and 'therefore' may also be used in explanations that are not intended to provide reasons for acting or believing something:3

► Since we forgot to add yeast, the bread didn't rise.

► We forgot to add yeast, therefore the bread didn't rise.

► We forgot to add yeast, thus the bread didn't rise.

► We forgot to add yeast, so the bread didn't rise.

5 Again, as with conclusions, a text or speech may not include specific premise indicators. Context is the best means of identifying premises in such cases. It may also help to try adding premise indicators to propositions to see if the passage or speech still runs smoothly.

6 Ordinary language can make identifying arguments more difficult than it might otherwise be because people do not always express all of their premises explicitly. Thus many attempts to persuade by argument rely on implicit premises. In Chapter 5 we will discuss the interpretation of hidden premises and the reconstruction of arguments to include them.

Intermediate conclusions

The conclusion of one argument may serve as a premise of a subsequent argument. The conclusion of that argument may itself serve as a premise for another argument, and so on. A simple illustration:

Fido is a dog. All dogs are mammals, so Fido is a mammal. And since all mammals are warm-blooded, it follows that Fido is warm-blooded.

In this argument, an intermediate conclusion - that Fido is a mammal - is used as a premise for a further argument, whose conclusion is that Fido is warm-blooded. We represent extended arguments of this kind like this:

3 While reading this book you may also have noticed a further use of 'thus1. 'Thus' can be used to mean 'in this way' and often precedes an example or a quotation.

P2) All dogs are mammals.

C1) Fido is a mammal.

P3) All mammals are warm-blooded.

C2) Fido is warm-blooded.

We give the two conclusions numbers: CI is the conclusion of an argument whose premises are PI and P2; C2 is the conclusion of an argument whose premises are CI and P3. So CI is both the conclusion of one argument and the premise of another.

Normally, in such cases, the last conclusion reached (the one with the highest number) is the proposition which the arguer is most concerned to establish. It is the ultimate target. So we call this simply the conclusion of the argument, whereas any other conclusions, reached as steps along the way, are called intermediate conclusions.

We sometimes want to concentrate for a moment on a particular part of an extended argument. In the above case, for example, we might be particularly interested either in the first part of the argument, or in the second. We will sometimes speak of the argument from PI and P2 to CI, or of the argument from CI and P3 to C2. We can also speak of the inference from PI and P2 to CI, and the inference from CI and P3 to C2.

The use of the word 'inference' in logic and critical thinking is another case where a word is used in a somewhat restricted sense in comparison with ordinary language. All reasoning consists of inferences, in the logician's sense of the word - each step of reasoning, each jump from premise or premises to conclusion, is an inference. Contrary to the way the word is often employed ordinarily, there need be nothing doubtful about an inference. We sometimes say, 'but that's just an inference', meaning to cast doubt upon whether a given proposition should really be accepted on the basis of others. But in our sense of the word, an inference may be completely certain, not subject to doubt. For example it is an inference, in our sense, to go from 'John is a classical musician' to 'John is a musician' - despite the fact that there can be no doubt that if the first proposition is true, then so is the second (in the terminology to be introduced in Chapter 2, it is a valid inference).

Linguistic phenomena

As we've seen, once we've determined that a text or a speech contains an attempt to persuade by argument, the remainder of argument reconstruction is largely a matter of interpreting the speech or text as accurately as possible. Here we are trying to work out what the speaker or writer intends readers or listeners to understand and consequently do or believe on hearing or reading their words. Phenomena in ordinary language sometimes make this task more difficult because they obscure speakers' and writers' intended meanings and therefore make it difficult to tell which proposition their sentences are supposed to convey. So aspirant critical thinkers need to be aware of the ways in which language can work to hide writers' and speakers' meanings and must practise spotting potentially problematic sentences. At this stage you should aim to be able to recognise these sentences and to be able to give the possible interpretations of them; that is, the propositions that they could be used to convey.

Ambiguity

A sentence is ambiguous in a given context when there is more than one possible way of interpreting it in that context - that is, if there is more than one proposition it could plausibly be taken to express in that context. There are two types of ambiguity.

Lexical ambiguity

This is a property of individual words and phrases that occurs when the word or phrase has more than one meaning. The set or group of things to which an expression applies is called its extension (it helps to think of an extension as all the things over which the word or phase extends or spreads itself). Thus the extension of the word 'student' is the set of all students. An ambiguous word or phrase, then, has two or more separate and different extensions - it picks out two or more different sets of things. Ambiguous words and phrases can bring their ambiguity into sentences, making those sentences capable of having more than one possible interpretation. The word 'match' is one such word. The sentence 'He is looking for a match' could be intended to mean any of the following propositions:

► He is looking for a small stick of wood with an inflammable tip.

► He is looking for another one the same [as this one],

► He is looking for [wants] a game of tennis (or some such).

Notice that it is not only nouns that can be lexically ambiguous. Suppose you are going to meet someone for the first time and all you've been told about them is what a friend has told you - 'She's a hard woman'. This could mean: She's a difficult person; She's an aggressive person; She has a very well-toned, muscular body. Whichever interpretation you adopt will have an important effect on your expectations of the woman in question. When interpreting sentences that are lexically ambiguous, we have to focus on the context in which they are written or said and the consequent probability of each of the possible interpretations being the correct one. For instance the sentence 'A visitor to the zoo was attacked by the penguins' is lexically ambiguous because the preposition 'by' has two possible meanings in this context. The sentence could express either of the following propositions:

► The penguins attacked a visitor.

► A visitor was attacked beside the penguins' enclosure.

However, in the absence of any information about a vicious penguin, and given what we know about the usually non-aggressive behaviour of penguins towards zoo visitors, it would probably be reasonable to interpret the sentence as intended to express the second proposition.

There are a few words that are not really ambiguous but may seem so when we hear them, though not when we see them written. This is because the words, though spelt differently, sound the same. For example when heard, as opposed to read, the question 'Are you a mussel (muscle) man?' could be either an enquiry as to someone's taste in seafood or as to his physique. Of course, once we see the question written, we are in no doubt as to its meaning.

The examples considered so far are relatively simple to understand because the alternative meanings of words such as 'match' and 'hard' are very different. However, instances of lexical ambiguity also occur when a word has alternative meanings that are much closer together. Such cases are much harder to interpret and we need to pay a lot of attention to the context in which the word is being used and to the probability of the speaker or writer intending one interpretation rather than the other.

Suppose someone argues, 'Fewer women have the ability to do complex mathematics than men'. The speaker or writer might mean to say that men have a greater natural or innate capacity for complex mathematics than women; that is they might want to express the idea that if the same number of women as men were to study complex mathematics under the same conditions, more men than women would succeed. If that were the intended interpretation, the claim would be sexist if it unjustifiably assumed that gender is a biological condition for success in mathematics. On the other hand, the speaker or writer might have intended to claim that as a matter of actual fact, there are fewer women than men successfully working at complex mathematics. This is true, but it is much less likely to be intended or interpreted as sexist: the relevant statistics are easily obtainable, and indeed might be cited by opponents of sexism as evidence that social factors encourage men, but discourage women, from specialising in mathematics.

The ambiguity here is due to the use of the word 'ability'. 'Ability' can refer to one's natural potential to do something - a potential with which one is born, and which may require training before engendering the actual capacity to do the thing - or it can refer to an actual capacity immediately to do the thing, a capacity which one may have acquired partly or wholly by training or practice. Often, it is not clear which meaning the speaker or writer intends. As you can see, the two ways of interpreting the word 'ability' are not entirely unrelated, as they are in the case of 'match'. Nonetheless, correct interpretation of which meaning a speaker or writer wishes to convey is crucial in determining how one should evaluate and respond to their remark.

Syntactic ambiguity

This occurs when the arrangement of words in a sentence is such that the sentence could be understood in more than one way (as expressing more than one proposition.) You will probably be familiar with examples of syntactic ambiguity as it is often the basis of jokes and newspaper headlines that appear odd. For example '33-year old Mrs Jones admitted to dangerous driving in Leeds Crown Court yesterday.' could mean either of the following:

► In Leeds Crown Court yesterday Mrs Jones admitted to dangerous driving.

► Yesterday Mrs Jones admitted to driving dangerously inside Leeds Crown Court itself.

The sentence is syntactically ambiguous because it could, consistently with English grammar, be used to express either proposition. But since the second interpretation is extremely unlikely, it is unlikely that an actual use of this sentence would be ambiguous. But consider this case:

Why should we become critical thinkers?

US President Bush has cancelled a trip to Scotland to play golf.

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