Criteria for evaluating arguments
So far we have looked at the different parts of arguments, how to distinguish arguments from nonarguments, and discussed the different types of arguments. It is now time to return to a topic we touched on earlier in the semester; how to evaluate an argument. In order to do this, we need a set of criteria. The criteria used for evaluating deductive arguments is going to differ from the criteria used to evaluate inductive arguments, because of the differences between deductive and inductive reasoning.

Valid arguments
We will begin by looking at the criteria for evaluating deductive arguments. Let us begin with the notion of validity. Deductive arguments can be said to be valid or invalid. A valid deductive argument is one where if the premises are true, then the conclusion is necessarily true. In other words, in a valid deductive argument, the premises support the conclusion. An invalid deductive argument is one where even if the premises are true, the conclusion is not necessarily true. In an invalid argument, the premises do not support the conclusion.. Much of our work this semester will be aimed at determining the validity or invalidity of arguments. Validity and invalidity are not matters of degree. An argument is either valid or invalid. There is no such thing as a slightly valid argument.

You need to notice something about our definition of validity: a valid argument need not have true premises. Because of this, it follows that the premises of a valid argument can be false, and so can the conclusion. For example:

Madonna is from England.
All people from England have red hair.
Therefore, Madonna has red hair.

This is a valid argument. The premises support the conclusion: if those premises were true, the conclusion would necessarily be true. The premises, though, are quite obviously false.

Validity, then, is not a judgement about the content of an argument. Rather, it is a judgement about the logical form of an argument. If one says an argument is valid, one says nothing about whether the argument’s content is true or false. Recall that back in section 1.1 you were told to ask two questions when analyzing an argument:

1) Are the premises true?
2) Do the premises support the conclusion?

Validity is a concept that deals only with the second of these questions. To say an argument is valid is to say only that the premises support the conclusion: that if the premises are true, the conclusion will also be true. A valid argument will never have true premises and a false conclusion. An argument with true premises and a false conclusion must be an invalid argument. Why? Think of it this way: arguments can be bad in one of two ways. Either they use bad information-- have false premises-- or use bad logic. Now, consider our argument with true premises and a false conclusion. It must be bad: the false conclusion tells us that. But it doesn't have bad content: the premises are true. So, that means the logic must be bad. A deductive argument with bad logic is termed invalid.

Valid arguments, though they cannot have true premises and a false conclusion, can have true premises and a true conclusion, false premises and a true conclusion, or false premises and a false conclusion. Invalid arguments can come in any of these four possible forms.

Soundness
To say that an argument with false premises and a false conclusion is valid may bother you, because it seems you are saying the argument is good. Yet clearly there is a problem with the argument: the content is false. Keep in mind that validity passes no judgement on the content of an argument. When you judge an argument to be valid, you are not saying it is good. The argument could still have a false premise.

Because of this, we need another criterion by which to evaluate both the content and the logic of a deductive argument. This is the criterion of soundness. A sound argument is an argument that meets two conditions:

1) It is valid
2) It has true premises

Any argument which does not meet both of these conditions is unsound. Unlike validity, then, soundness is not a judgement that applies simply to logical form: it is also concerned with the argument’s content. It takes into account both of our questions of argument analysis, for it asks whether the premises are true and whether those premises support the conclusion. A sound argument is the ideal type of deductive argument: it has a valid logical form and true content. Such an argument will have a necessarily true conclusion. Sound arguments are what one should strive for in a deductive argument. An example:

Bruce Springsteen lives in New Jersey.
New Jersey is near New York.
Therefore, Bruce Springsteen lives near New York.

To say that an argument is sound is to say that both of our questions of argument analysis are answered "yes." Yes, the premises are true, and yes, the premises support the conclusion. If you accept an argument as sound, you should accept its conclusion. A sound argument is a good argument.

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