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Critical Thinking By Example

 Chapter 8: Accepting Premises: Considerations of Logic and Language
  Quiz 8.1 Quiz 8.2


Material covered in this chapter

  • 8.1 Fallacies in accepting premises

  • 8.2 Logical considerations in accepting premises

  • 8.3 Considerations of language in accepting premises


Recall that we said that with good arguments it is possible to answer “yes” to each of these questions:

1. Assuming that the premise set is acceptable, is the premise set relevant to the conclusion?

2. Assuming that the premise set is acceptable, does the premise set provide sufficient support for the conclusion?

3. Is the premise set acceptable?

A bad argument is one that answers “no” to one or more of these questions. In this chapter and the next we will examine arguments where the answer to question 3 is “no”. Since question 3 is so broad, I propose that we subdivide it into these two questions:

 Is the premise set logically and linguistically acceptable?

 Is the premise set evidentially acceptable?

In this chapter we will deal with the former question. I will first list some fallacies, and then discuss some general strategies for thinking about whether premises are logically and linguistically acceptable.

8.1 Logical and Linguistic Fallacies in Accepting Premises


Fallacy of Begging the Question

This fallacy is present when the conclusion is the same as a premise.

In example 8.1, the conclusion and the premise are the same despite the different wording: ‘Darwinianism’ is simply another name for the ‘theory of natural selection’.

Example 8.1

“You should believe in Darwinianism because the theory of natural selection is true.”


Fallacy of False Dilemma

This fallacy is present when a premise fails to mention all the relevant alternatives.

In example 8.2 the author conveniently neglects the alternative that he or she may be the guilty party.


Example 8.2

“Detective Colombo: here is what happened. Only five of us were on the island: me, the victim, Andy, Betty and Charlie. So, the guilty party must be Andy, Betty or Charlie.”

Fallacy of Equivocation

This fallacy is present when an argument appears successful only because of an ambiguous premise.

In example 8.3, the premise only appears to adequately support the conclusion because of the ambiguity of the word ‘bank’. In one sense it may refer to a type of financial institution, in another sense it refers to the land adjacent to rivers. Presumably the radio report refers to the latter sense of the term, and the conclusion about wet money to the former sense of the term.

Example 8.3

“I heard on the radio that the rains have swelled the river so much that the banks have been flooded. So, your money in the bank must be soaked.”

Fallacy of Vagueness

This fallacy is present when an argument appears successful because of the vagueness of the premise set.

In example 8.4, the term ‘bald’ is vague. It can mean anything from some loss of hair, to having no hair on one’s head. Since the premise is not more precise, it does not tell us whether the actors have some or no hair, we can’t adequately assess whether the premise supports the conclusion.

Example 8.4

“The intern said that all the actors auditioning are bald. So, we won’t need to worry about how their hair looks, since they have none.”

Fallacy of Persuasive Definition

This fallacy is present when a term is surreptitiously redefined to make an argument appear stronger.

In example 8.5, the author redefines the word ‘anarchist’ without letting the audience know that the term has been redefined. (Normally it refers to the view that there is no legitimate political authority).

Example 8.5

“We are all anarchists because anarchists are people who break the law, and we have all broken the law at one time or another. For example, who among us can say that they have never gone over the speed limit?”


8.2 Logical Considerations in Accepting Premises


One logical consideration in accepting premises is the prescription that we ought to reject circular arguments. It is perhaps best to explain why we shouldn’t accept circular reasoning with an example. Suppose I argue, “Socrates is the greatest teacher because Socrates is the greatest teacher.” If you doubt the conclusion, “Socrates is the greatest teacher”, then you have reason to doubt the premise, “Socrates is the greatest teacher”.  After all, the premise and the conclusion are the same; the premise adds nothing in way of support of the conclusion. On the other hand, if you believe the premise, then you have no additional reason to believe the conclusion, since the conclusion and the premise are one and the same. Rarely do circular arguments appear in such an obvious fashion; often the premise and conclusion express the same idea with different words. In the above example, the premise might be stated differently as: “No teacher equals or exceeds Socrates in terms of teaching ability.” The circularity in this instance would be a little less obvious, but the argument still commits the fallacy of begging the question, since the conclusion and premise express the same idea in different words.

As we have said, arguments attempt to give us reasons to believe or do something. Whether we ought to believe or do X will often depend on how plausible the alternatives are, and arguments that do not consider all the alternatives, in effect, rig the outcome. The associated fallacy is called “False Dilemma”, because often arguments that commit this fallacy offer only two choices when there are in fact more options that should be considered, e.g., either you are a friend or an enemy, either you are a Republican or Democrat, either you are an atheist or a theist. In these instances there are other alternatives that might be considered: you might be an acquaintance, a social democrat, or agnostic.

In some cases there are only two relevant alternatives. The premise, either you are a friend or you are not a friend, covers all the possibilities. For not being a friend covers the case where you are an enemy, an acquaintance, or unknown to me. Note too that just because more than two alternatives have been given in an argument does not mean that all the relevant alternatives are present. In example 8.2 the author mentions three possible suspects but neglects to mention him or herself as a possible suspect.

8.3 Considerations of Language in Accepting Premises

Turning to considerations of language, it is important to note that almost every argument will contain instances of vague and ambiguous language. The reason is that arguments are made in human languages, and human languages are brimming with vague and ambiguous terms. For example, most terms that refer to material objects in our environment are vague. You might think the term ‘chair’ is not vague, since it is clear when something is a chair, and when it is not: we are not likely to confuse a chair with a refrigerator, for example. But think about this. Imagine your favourite four legged chair and then imagine cutting a centimetre off of each leg of the chair. Is it still a chair? Probably. Now imagine cutting another centimeter, and another centimeter, and each time asking whether it is a chair. At some point it may be hard to say whether it is a chair or not. It is not clear where exactly something stops being a chair. The same is true of ambiguity. If you flip through a dictionary, you will see that many terms have multiple meanings.

So, it should not be surprising then that the mere presence of vagueness or ambiguity is not sufficient to demonstrate that the fallacies of vagueness or equivocation are present. Consider this argument: “John has no hair, so John is bald. The announcer said that anyone who is bald is eligible for a prize. So John is eligible for a prize.” The fact that a premise of this argument, John is bald, is both vague and ambiguous does not make this a bad argument. The term ‘bald’ is vague because it is not clear where baldness starts. Despite this vagueness, there are clear examples of when someone is bald, including John: he has no hair. The term ‘bald’ is also ambiguous as it can mean (among other things) ‘forthright’ or ‘blunt’. It would be very uncharitable to think that the author is arguing that John is blunt because he has no hair. So, although ‘bald’ is ambiguous, this ambiguity presents no problem in the above example. Again, vagueness and ambiguity are common phenomena, and the associated fallacies only come into play when these phenomena are exploited to make a weak argument appear stronger.

Since language is often vague and ambiguous, arguments will sometimes employ definitions to clarify matters. There are different types of definitions. A reportive definition seeks to report how a term is commonly used; dictionaries are prime examples of repositories of reportive definitions. They tell us how words are typically used. Two common problems with reportive definitions is that they are too wide or too narrow. A reportive definition is too wide if it includes individuals that are not normally included under the term. A definition is too narrow if it excludes individuals that are normally included under the term. Consider this example: the definition of “bachelor” is “an unmarried person”. This definition is too wide because if it were true, then it would include females as part of the definition of bachelors. Since no female is a bachelor, the definition is too wide. Now consider the definition an “unmarried person” is a bachelor. This definition is too narrow: it excludes females. Females are among unmarried persons. It sometimes helps students to visualize definitions as lassoes. A good reportive definition of X will lasso all and only the things that X refers to. In the first example, the definitional lasso “bachelor means unmarried person” captures females in the lasso. The lasso is too wide, the definition captures too much, because it captures females. In the second case the lasso is too narrow, because it captures only males. A good definition of “unmarried person” should also capture females.

Sometimes a definition can both be too wide and too narrow. Consider this definition: “A Professor is someone who gives final exams.” This lasso is too wide because it will capture some non-professors, e.g., high school teachers often give final exams, but they are not professors. On the other hand, the lasso is too narrow because it excludes those professors who do not give their classes final exams. (I’m told this happens). So, this definition is both too wide and too narrow.

Stipulative definitions are redefinitions of existing terms, or definitions for newly introduced terms. Often a stipulative definition is used to clarify ambiguity or vagueness in our ordinary terms. In some legal jurisdictions, for example, a ‘child’ is defined as a person under the age of 21. This clarifies the vagueness of the term ‘child’ as it is used in common parlance: it is not clear at what age someone stops being a child in our ordinary use of the term. It also clarifies an ambiguity: one sense of ‘child’ means ‘offspring’, e.g., “I am my mother and father’s child.”

Persuasive definitions pass themselves off as reportive definitions when in fact they are disguised stipulative definitions. In example 8.5, it looks like the author has reached a rather remarkable conclusion: we all unwittingly hold a very unpopular political philosophy: anarchism. Once we realize that the author has stipulated a new meaning for ‘anarchism’ the conclusion is not particularly impressive.