There are claims, and then there are claims with supporting arguments. Chapters 8–12 fill out the work of critical thinking by doing for arguments what we have already done for claims: spotting them, identifying their parts, and finally evaluating them.

The evaluation of arguments plays a relatively small role in Chapter 8, just as the evaluation of claims played only a small role in Chapter 1's account of what a claim is. But it is even truer of arguments than it was of claims that you will not get far in judging their merits until you have first gotten clear on what they are and how they should work. This chapter sets the project in motion by showing how to clarify and analyze an argument. Arguments will be divided into two broad groups, deductive and inductive, dissected into their essential parts—mainly, premises and conclusions—and characterized as strong or weak, valid or invalid, depending on how well the premises lead to the conclusions.

1. An argument comprises a conclusion and premises.
2. We evaluate arguments with the terms good or bad, valid or invalid, sound or unsound, strong or weak.
3. By understanding what would make an argument good, we can identify and evaluate its unstated premises.
4. The final step before evaluating an argument is understanding it.
5. The entirety of critical thinking comes into play when evaluating an argument. You need to ask whether the premises are likely to be true, and whether they support the conclusion.

1. An argument comprises a conclusion and premises.

1. The conclusion is the claim that the argument intends to support, and the premises are all the claims offered in support of the conclusion.
2. An argument may leave a premise, or even its conclusion, unstated.
1. "[Conclusion] You don't want to shop at Lucky's Car Stereo. [Premise] A man has to court disaster to win the name 'Lucky.'" Unstated premise: You don't want to shop at a car stereo business owned by a man who courts disaster.
2. "[Premise] If old age were bad in itself, every old person would be unhappy. But [premise] I'm old and I'm happy." Unstated conclusion: Old age is not bad in itself.
3. An argument may contain either dependent or independent premises supporting a single conclusion.
1. Dependent premises need one another all to be true, to make the argument work.
1. The argument then works because the premises all connect with each other in the right way.
2. "[Premise] Joe is a chiropractor, and [premise] chiropractors are experts about human bodily movement, so [conclusion] Joe must be an expert about human bodily movement."
2. Independent premises work individually even when one of them is false.
1. You may think of an argument with two independent premises as two arguments for the same conclusion.
2. "[Conclusion] I didn't dent your car door. First of all, [premise] I never borrowed your car. In the second place, [premise] the door was fine when I returned the car to you. And anyway, [premise] the door was dented when I borrowed the car."
3. No sane person would ever make that argument, but note that, logically speaking, even if one or two of the premises are false, we have some support for the conclusion as long as at least one is true.

2. We evaluate arguments with the terms good or bad, valid or invalid, sound or unsound, strong or weak.

1. At the most general level we call an argument good if it gives grounds for accepting its conclusion.
1. "Good" and "bad" are relative words when applied to arguments: It is often more apt to call one argument better than another than to call one absolutely good.
2. The following discussion will elaborate the grounds for calling an argument good or bad.
2. When premises provide undeniable support for the conclusion, the argument is valid and perhaps also sound.
1. An argument is valid if, whenever all its premises are true, the conclusion is true as well.
1. More casually, you may say that the conclusion of a valid argument must follow from its premises.
2. The premises do not all have to be true for an argument to be valid. So validity does not guarantee a true conclusion. But if all the premises of a valid argument are true, then the conclusion is true.
3. An argument that is intended to be valid, but isn't, is invalid. (This sounds too obvious to say. It is not. The point is that only arguments intended as valid arguments should be evaluated in terms of their validity or invalidity.)
2. An argument is sound if it is valid and all its premises are true.
1. That means that the conclusion follows from the premises, and that all the premises are true.
2. Given the definition of validity, it follows that a sound argument always has a true conclusion.
3. An argument is unsound if any of its premises are false or if it is not valid.
3. Whereas a valid argument is not necessarily a good one (because of the possibility of false premises), a sound argument normally is good.
3. When arguments are not intended to count as valid, they may be evaluated as strong or weak.
1. An argument is strong if, whenever all its premises are true, the conclusion is unlikely to be false.
2. Good examples of strong arguments are those that rest on prior experience or observation.
4. We may now describe a good argument more precisely as one that is either sound or strong.
1. Validity and invalidity are absolute terms—they either hold of an argument or they don't. But strength and weakness are relative, depending on the relative likelihood of the argument's conclusion being true.
2. For this reason, the two sets of terms work better when kept separate: Do not call valid arguments strong or weak, or call strong or weak arguments valid.
5. We can handily distinguish deductive and inductive arguments using this vocabulary.
1. Deductive arguments are valid, or intended to be valid; inductive arguments are neither.
2. Hence, deductive arguments are those we evaluate as valid or invalid, sound or unsound, whereas inductive arguments are evaluated as strong or weak.

3. By understanding what would make an argument good, we can identify and evaluate its unstated premises.

1. Many arguments encountered in daily life contain unstated premises.
2. An unstated premise is an assumption that, when added to the stated premises, makes the argument either valid or strong.
1. The argument "My car won't start, so the starter wires must have shorted" becomes valid if we add "Whenever a car does not start, its starter wires have shorted." But that added assumption seems unlikely to be true.
2. The same elliptical argument becomes strong with the added assumption "Lately my car has not been starting, and each time the starter wires had shorted."
3. The second added assumption has much more plausibility, so we should go with that one and produce a strong argument rather than a valid one.
3. The merit of the argument depends on the plausibility of these added assumptions.
1. When an argument needs false or unlikely assumptions to become good, it is not a good argument.
2. It is also not a good argument if reasonable added assumptions leave it invalid or weak.
4. Hence you should respond to an incomplete argument by looking for a reasonable claim to add to its existing premises.
1. Find a general claim that would make the argument valid.
2. If that effort leaves you with an implausible premise, modify the claim to make the argument strong.
3. Reject an incomplete argument when no reasonable claim will make it either valid or strong.
5. Although the identification of missing steps in an argument makes for fine reading, fine writing calls for providing all those steps to begin with.

4. The final step before evaluating an argument is understanding it.

1. Evaluation applies to both the argument's claims taken separately and their function in the context of the argument. We ask two questions:
1. Are the premises reasonable—that is, probably true?
2. Do the premises form a good support—that is, a valid or strong argument—for the given conclusion?
2. Techniques of clarification repair the most common problems of understanding.
1. Arguments become difficult to understand when they go by quickly, exhibit a complicated structure, surround themselves with nonargumentative material, or have no clear structure or no good premises.
2. In each case, the argument needs to have its premises and conclusion identified, and the interrelationships among them spelled out.
3. A simple method lets you render arguments in diagrams that clarify their structure.
1. Circle all premise and conclusion indicators—all words that show that one claim depends on another: "for," "since," "hence," "therefore," "it follows that," and so on.
2. Bracket each premise and conclusion and number them consecutively.
3. Arrange the numbers for these claims, with the following symbols to show their interconnections:
1. An arrow from one number to another means that the first is supposed to support the second.
2. A line under two numbers, with a plus sign between them, shows that the two are dependent premises. An arrow then goes from the line to the claim they support.
3. Separate arrows from different numbers or groups of numbers indicate independent premises.
4. When a claim supports more than one conclusion, an arrow goes from it to each claim it supports.
5. An arrow from one number to another with lines through it means that the argument has included this first claim as a counterargument to its conclusion.
4. Nonargumentative material, or window dressing, often makes this structure harder to see.
1. The passage may not be an argument at all, but a description or explanation. Ask: Does the speaker offer reasons for the stated claim?
2. The passage might not state its conclusion.
3. When no premises seem to be present, the passage may be a piece of nonargumentative persuasion (see Chapter 4) or contain pseudoreasoning (Chapters 5 and 6).

5. The entirety of critical thinking comes into play when evaluating an argument. You need to ask whether the premises are likely to be true, and whether they support the conclusion.

1. To determine whether unsupported premises in an argument are reasonable, use the methods covered in Chapters 1–7. To review:
1. Claims should possess initial plausibility and come from a credible source.
2. Except when there are very good other reasons for accepting them, claims should not conflict with observation or background information.
3. Claims from one credible source should not conflict with claims from another: Accept neither before resolving the question of which source to believe.
4. Ambiguous, vague, or otherwise unclear claims should be clarified before being accepted.
2. Bear in mind that although two arguments may both have reasonable premises, those in one argument can be more probably true than the premises of the other. Then the first argument is better.
3. To determine the validity or strength of an argument, use the methods to be covered in Chapters 9–1.
1. In the case of inductive arguments, their relative strength depends on more than the likely truth of their premises. See Chapters 11 and 12.