The Language of Argument

9 3

so m e st a n d a r d s f o r ev a l u a t i n g A r g u m e n t s

Now compare this argument:

(1) Either Joe or Jack or Jim or Jerry committed the murder.

(2) Joe did it.

∴(3) Jerry did not commit the murder. (from 1–2)

To show that this argument is invalid, all we have to do is explain how the

premises could be true and the conclusion false. Here’s how: Joe and Jerry

did it together. In that case, Jerry did it, so the conclusion is false; Joe also

did it, so the second premise is true; and the first premise is true, because it

says that at least one of these four suspects did it, and that is true when more

than one of the suspects did it. That possibility of complicity, thus, makes

this argument invalid.

We will explore many more forms of argument in Chapters 6 and 7. The

goal for now is just to get a feel for how to determine validity. In all of these

examples, an argument is said to be valid if and only if there is no possible

situation in which its premises are true and its conclusion is false. You need

to figure out whether there could be any situation like this in order to deter-

mine whether an argument is valid. If so, the argument is invalid. If not, it

is valid.

This definition shows why validity is a valuable feature for an argu-

ment to possess: There can be no valid argument that leads one from true

premises to a false conclusion. This should square with your commonsense

ideas about reasoning. If you reason well, you should not be led from truth

into error.

What are known as deductive arguments are put forward as meeting this

standard of validity, so validity is one criterion for a good deductive argu-

ment. Other arguments—so-called inductive arguments—are not presented

as meeting this standard. Roughly, an inductive argument is presented as

providing strong support for its conclusion. The standards for evaluating

inductive arguments will be examined in Chapters 8-10. For now, we will

concentrate on deductive arguments.

Truth

Although a deductive argument must be valid in order to be a good argu-

ment, validity is not enough. One reason is that an argument can be valid

even when some (or all) of the statements it contains are false. For example:

(1) No fathers are female.

(2) Sam is a father.

∴(3) Sam is not female. (from 1–2)

Suppose that Sam has no children or that Sam is female, so premise 2 is false.

That would be a serious defect in this argument. Nonetheless, this argument

satisfies our definition of validity: If the premises were true, then the conclu-

sion could not be false. There is no way that Sam could be female if Sam is a

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