2.8: Evaluating Arguments

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We do much of our reasoning almost automatically, so it is easy to overlook how frequently we engage in it. Any time someone gives reasons to support a claim, they are giving an argument. Much of this textbook is devoted to the evaluation of arguments, and we will find three key issues that surface over and over again.

Once you have identified an argument, you must ask three questions.

Are the premises true (or at least plausible)?
Has any relevant information been omitted from the premises?
Do the premises support the conclusion?

Are the premises plausible?

If the issue is whether you should believe the conclusion, then the first question to ask is whether the premises are plausible. In this context, nothing can salvage an argument if one or more of its premises are false. If the premises of an argument—even just one of them—are false, we have no reason to accept its conclusion.

Sometimes we can’t be certain whether the premises of an argument are true, and we must settle for plausibility instead. But the more plausible the premises, the better. If you are simply doing what-if reasoning, the plausibility of the premises may not matter; you are just asking what would be true if the premises were true, and in this context, it doesn’t matter whether they are actually true.

Has relevant information been omitted?

When it comes to reasoning, ignorance is not bliss; what you don’t know can hurt you. An argument may have all true premises and yet omit information that is relevant to our evaluation of it. Suppose Wilbur tells you that Jack would be a good person to buy a used car from because Jack knows a lot about cars and doesn’t use high-pressure techniques. These premises may be true, but if Wilbur fails to mention that Jack has done time for fraud, you are in trouble if you accept the conclusion of his argument.

We can’t usually get the whole truth and nothing but the truth; examining all the evidence that might conceivably be relevant would be an endless task. But we should never neglect evidence that we know about, or evidence that seems like it might bear on the issue in a major way.

In many cases, we need a good deal of background knowledge to answer the first two questions. If the argument is about football, we need to know something about football; if it is about cooking, we need to know something about cooking. Logic can’t supply this information, but we will discuss various factors, e.g., evaluation of sources, that can help us answer the first two questions when they arise in real life. We will see that this question doesn’t arise when we are evaluating a deductively valid argument.

How strongly, if at all, do the premises support the conclusion?

This is another way of asking whether the argument is deductively valid or inductively strong and, if it’s the latter, just how strong it is.

To master various key concepts, we will sometimes focus on one of these questions without worrying about the others. But when we put things all together at the end, when you are evaluating reasoning in the real world, all three questions are important.