Statements, Arguments, Validity, Soundness and Informal Fallacies

STATEMENTS

(Sometimes called 'Proposition')

A statement is something that may be true or false. Consider the following:

I am pretty sure that the statement expressed by the first sentence above is true and I am pretty sure that the statement expressed by the second sentence is false. I am uncertain as to the truth or falsity of that statement expressed by the third sentence.

We should not confuse statements and the linguistic expressions (usually sentences) used to express or make statements. The following two sentences express the same statement.

We can say the same about the following two sentences:

The following linguistic expressions do not express statements:

A little thought will convince you that the question of truth or falsity cannot be at issue here.

ARGUMENTS

The use of the word 'argument' in:

is not the use we have in mind here. As we shall use 'argument' in this course, it is not synonymous with 'dispute' or 'debate' nor even with 'discourse' or 'rational discussion'. In our sense of the word, arguments may occur in, but are not the same thing as disputes, debates, discourses, and discussion. There is nothing wrong with using 'argument' as it is used in the indented sentence above. But it is not that use in which we are interested.

What we shall mean by an argument can be stated concisely as follows:

In giving an argument, a person claims that the conclusion is true and attempts to justify this claim by giving reasons (the premises) for the claim. That is to say, in giving an argument, we make a claim (the conclusion) and give reasons for the claim (the premises). We argue from the assumed truth of the premises to the truth of the conclusion.

The following is an argument:

The word 'therefore' designates (3) as the conclusion. (1) and (2) are premises.

The argument just given is in what we shall call standard form. By putting an argument in standard form we start to uncover its logical form. To reveal an argument's logical form it is necessary to separate its conclusion from its premises. Sometimes we do this simply by drawing a line to indicate that the statement below the line is the conclusion, while the statement(s) above the line is (are) the premise(s).

Needless to say, arguments are usually not presented in standard form. Thus, we might find the arguments just given presented as follows:

OR:

I have found that the most difficult part of evaluating an argument contained in a passage of ordinary prose is discovering exactly what the argument is, which is to say discovering what it would look like in standard form. Once we have an argument in standard form, its evaluation is often relatively easy.

VALIDITY

It has been my experience that the notion of validity is a difficult one for the beginning student to grasp and, once grasped, easily forgotten. So pay close attention.

OR:

OR:

It is important that you not forget the 'must, 'impossible', 'necessity' in these definitions. They are essential. Thus, it is not sufficient for validity that the premises and conclusion are all in fact true, nor is it sufficient for invalidity that a, some, or all the premises are false. It is sufficient for invalidity that the premises are in fact true and the conclusion is in fact false because that is precisely what we said was impossible if the argument is to be valid. But more importantly, an argument is invalid if it could be the case (even though it may in fact not be the case) that the premises are true while the conclusion is false.

Look again at the argument in standard form concerning Red Dye #2. This is a valid argument. It is impossible that (1) and (2) are both true while (3) is false. If you do not see that this is the case, try supposing that (1) and (2) are true and (3) is false. If (3) is false, then it is not the case that Red Dye #2 should be banned by the FDA. Now either Red Dye #2 causes cancer or it does not. If it does, then (1) is false, for there is at least one thing, namely, Red Dye #2, that causes cancer and yet should not be banned by the FDA. But this contradicts our supposition that (1) is true. Then it must be that Red Dye #2 does not cause cancer. But this contradicts our supposition that (2) is true. Hence, we cannot without contradicting ourselves suppose that (1) and (2) are true and (3) is false. But to admit this is to admit that the argument is valid. (See the second definition of validity).

Note, however, that showing the argument to be valid does not show that the premises are true nor that the conclusion is true. Rather, it shows only that if the premises were true, then the conclusion would have to be true also.

Now look again at the arguments concerning Red Dye #2 that are not in standard form. If we take these as stating in a different way the argument in standard form we have just discussed, then they are valid. But consider the second, i.e.,

If we suppose the standard form of this argument is

then the argument is not valid. For it is not impossible that the premise is true while the conclusion is false. because it is not impossible that some things that cause cancer should not be banned and that Red Dye #2 is one of these.

There are two things that need saying with respect to this last. First, it is unsympathetic to take the argument in this way. I think it is pretty clear that the author of this argument believes and implies that anything that causes cancer should be banned by the FDA. Second, if you think about it, you will see that in the end it does not really matter which of the standard forms you choose. Let us assume that you do believe that Red Dye #2 causes cancer but that you not believe that anything that causes cancer should be banned by the FDA. Then, although the first standard form we gave "makes the argument valid, you will not for that reason accept the conclusion because you think the second premise is false. On the other hand, suppose that you think that Red Dye #2 causes cancer and also believe that anything that causes cancer should be banned by the FDA. Then, even though the second standard form we gave is invalid, you will nonetheless accept the conclusion because the argument can be easily made valid by adding a premiss that you do accept.

Philosophers often say that validity is not a matter of the content of the argument -- i.e., what the statements are about (or even whether or not they are true) -- of the statements that make up an argument, but rather validity is a matter of the argument's "logical form." The logical form of an argument is independent of its content. For example, consider the following two arguments:

These two arguments have the same logical form, a form known as "Modus Tollens." It is possible to abstract from the content of these arguments by replacing the basic statements in them by capital letters (variables that stand for a single or "atomic" statement). The logical form of modus tollens is:

Another type of argument that is know to be valid is called "Modus Ponens." It takes the following form

Any argument that takes one of these two form will be valid no matter what its content is; that is, its conclusion must be true if its premises are all assumed to be true, no matter what the statements are about and no matter if the premises are in fact true. Validity is a function of logical form, not content: An argument is valid if and only if, if the premises are true, the conclusion must be true.

Now there are a number of basic argument forms that are valid and these can be combined into larger structures to make up long arguments. In particular, one argument may be used to prove a statement that is used as a premise for another argument. But throughout, the notion of validity remains the same.

SOUNDNESS

If an argument is valid and all of the premises are true, then we say the argument is sound. If an argument is sound, then it follows from the definition of soundness and validity that its conclusion must be true. A sound argument is a good argument in that it shows that the conclusion must be true.

We know that the conclusion of a sound argument must be true, but don't assume that the conclusion of an unsound argument is false: If an argument is sound, it gives us reason to believe that the conclusion is true. If an argument is unsound it simply fails to give us reason to believe that the conclusion is true. If the argument is unsound then the conclusion may be true, but it is not necessarily true.

For example, we can modify one of the valid arguments given above to produce a false conclusion:

This argument is valid (it has the logical form of modus tollens) but its conclusion is false. This is possible because at least one of the premises (in this case (2)) is false.

An invalid argument with a true conclusion:

An invalid argument with a false conclusion:

To sum up: An argument proves its conclusion only if it is sound, if it is not sound it fails to prove its conclusion.

It is also possible to talk about "degrees of certainty" in rational arguments. It is possible to make rational arguments in cases in which the premises are thought to be true, but not known to be true. In these cases, it is important to realize that the conclusion of an argument can never be known (through the argument alone) with a greater degree of certainty than the least certain premise. However, if the premises are not known with certainty, but are warranted on the basis of evidence, then the conclusion of a valid argument beginning with those premises is warranted to the same degree.

THE ETHICS OF RATIONALITY

If you believe that the premises of an argument are true and also believe that the argument is valid, then you should believe that the conclusion is true. That is, if you believe an argument is sound, than you should believe that the conclusion is true. To believe otherwise is to hold two inconsistent beliefs, at least one of which must be false. If you do this knowingly, then you may rightfully be branded irrational or perverse.

INFORMAL FALLACIES

A fallacious argument is a bad argument which tends to mislead. Now, arguments may be bad for different reasons--they might be invalid, they might be unsound, they might be unpersuasive. Arguments also may mislead in various ways -- they may beg the question, they may base themselves on insufficient evidence, they may rely on ambiguous language.

The largest and most natural division among fallacies is that between formal and informal fallacies. We have already discussed formal fallacies when discussing validity. Formal fallacies include such things as Affirming the Consequent, Undistributed Middle, etc. Now we want to deal with informal fallacies, and these turn out to be a more disparate group, much harder to sort out than the formal. In fact, there can be no determinate list of fallacies, since there are as many fallacies as there are ways for people to think up a misleading argument. The following list is an attempt to organize some of the common ones.

I. Begging the Question (Petitio Principii)

This is a general sort of fallacious move which involves pretending a particular point or premise has been granted when it has not.

II. Irrelevance

There are numerous ways in which fallacies can mislead by taking up irrelevant material. The terms ignoratio elenchi (missing the point) and non-sequitur (it doesn't follow) are frequently used to cover any and all of the fallacies of irrelevance.

III. Insufficient Evidence

These are sometimes listed as inductive fallacies. The basic error involves drawing a conclusion on insufficient grounds. Once more, any form of this is fallacious, but forms which occur frequently have received the following names:

IV. Ambiguity

The use of a word is an ambiguous use of the word when it is not clear from the context which of two equally acceptable senses of the word are intended.