The New Critical Thinking

An Empirically Informed Introduction

Jack Lyons, Barry Ward

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781134881888
• Edition: 1
• Topic: Philosophy
• Subtopic: Philosophy History & Theory

Why is it so hard to learn critical thinking skills?

Traditional textbooks focus almost exclusively on logic and fallacious reasoning, ignoring two crucial problems. As psychologists have demonstrated recently, many of our mistakes are not caused by formal reasoning gone awry, but by our bypassing it completely. We instead favor more comfortable, but often unreliable, intuitive methods. Second, the evaluation of premises is of fundamental importance, especially in this era of fake news and politicized science.

This highly innovative text is psychologically informed, both in its diagnosis of inferential errors, and in teaching students how to watch out for and work around their natural intellectual blind spots. It also incorporates insights from epistemology and philosophy of science that are indispensable for learning how to evaluate premises. The result is a hands-on primer for real world critical thinking. The authors bring over four combined decades of classroom experience and a fresh approach to the traditional challenges of a critical thinking course: effectively explaining the nature of validity, assessing deductive arguments, reconstructing, identifying and diagramming arguments, and causal and probabilistic inference. Additionally, they discuss in detail, important, frequently neglected topics, including testimony, the nature and credibility of science, rhetoric, and dialectical argumentation.

Key Features and Benefits:

• Uses contemporary psychological explanations of, and remedies for, pervasive errors in belief formation. There is no other critical thinking text that generally applies this psychological approach.

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• Assesses premises, notably premises based on the testimony of others, and evaluation of news and other information sources. No other critical thinking textbook gives detailed treatment of this crucial topic. Typically, they only provide a few remarks about when to accept expert opinion / argument from authority.

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• Carefully explains the concept of validity, paying particular attention in distinguishing logical possibility from other species of possibility, and demonstrates how we may mistakenly judge invalid arguments as valid because of belief bias.

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• Instead of assessing an argument's validity using formal/mathematical methods (i.e., truth tables for propositional logic and Venn diagrams for categorical logic), provides one technique that is generally applicable: explicitly showing that it is impossible to make the conclusion false and the premises true together. For instructors who like the more formal approach, the text also includes standard treatments using truth tables and Venn diagrams.

  
  
  

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• Uses frequency trees and the frequency approach to probability more generally, a simple method for understanding and evaluating quite complex probabilistic information

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• Uses arguments maps, which have been shown to significantly improve students' reasoning and argument evaluation

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PART I

Deduction

CHAPTER 1

Validity and Why It Matters

The main topic of this book is the proper evaluation of evidence. This means the proper evaluation of arguments.

An argument, again, is a set of sentences (or, as we’ll sometimes call them: claims, statements, or propositions) consisting of one or more premises and a conclusion. The premises are statements that are offered as evidence for the conclusion, and the conclusion is the statement whose truth the argument is intended to establish. Logicians typically distinguish between deductive arguments and inductive arguments. Roughly speaking, an argument is deductive if the truth of premises would guarantee the truth of the conclusion; an argument is inductive if the truth of premises would render the truth of the conclusion probable, without guaranteeing it. Some inductive arguments are very powerful, and the probability they confer is extremely high. There’s nothing wrong with an inductive argument just because it doesn’t absolutely guarantee its conclusion. Nevertheless, inductive arguments are messier and more complicated than deductive arguments. Thus, in this chapter and Chapters 2 and 3, we will focus on the stronger and simpler kind of argument, the deductive argument. Simple doesn’t mean easy, and the next two chapters will be a bit abstract, but please bear with us. The skills and concepts mastered here will be important for nearly all other reasoning.

Our provisional understanding of deduction is rough in two ways. First, we’ll want to say quite a lot more about what’s meant by “guarantee.” Second, if we were to define deductive and inductive arguments as those that guarantee or make probable their conclusions, it would follow that there couldn’t be bad arguments of either type, arguments that abjectly fail to provide the kind of support they’re intended to. Consequently, we’ll officially define deduction and induction in terms of the aims of the argument, that is, in terms of the intentions of the person offering the argument. Thus, we will define a deductive argument as one that aims at validity, i.e., one that purports to be valid. “Validity,” of course, is a technical term that replaces the more intuitive but less precise “guarantee.” Just what it means is the topic of this chapter. (We’ll say more about aims and intentions a bit in this chapter, but more so in Chapter 3.)

1. DISTINGUISHING THE GOOD FROM THE BAD

The goal here is to distinguish good arguments, ones whose premises provide a genuine reason to believe the conclusion, from bad ones. The good news: You already know a lot about how to do this. From an early age, we reliably use this ability on a daily basis. So, the task of this book is not to introduce some alien, intellectual discipline, but to develop and refine a skill you already possess. To see that we have this skill, take the following pair of examples:

• (P1) All members of species X have lungs.
• (P2) y is a member of species X.
• (C) Therefore, y has lungs.

• (P1) All members of species X have lungs.
• (P2) y has lungs.
• (C) Therefore, y is a member of species X.

The first is a good argument and the second is a bad one, and we confidently make that judgment. So, in some sense, we already know the difference.

The question now is: can we say what the difference is? What is it about the good argument that makes it good and the bad one that makes it bad? What is the relevant contrast between them, the difference that makes a difference?

What counts as a successful answer here? First, we want to know what makes arguments good in general, not just the first argument in particular. It is relevant and true to say, “the first argument is good, because “y has lungs” follows from y being a type X and all Xs having lungs,” but that answer is too specific. It does not tell us how to evaluate arguments about economics or physics or the likelihood of rain. There’s another problem with that answer. To say that the conclusion follows from the premises is correct, but unhelpful. If we can’t say what that means in simpler terms, saying “it follows” is no more illuminating than saying the argument is good. We haven’t explained what it is for the argument to be good. The same goes for saying that the conclusion is a consequence of the premises, or that the premises imply the conclusion, or, if you’ve already been exposed to some logic, that the argument is valid. All true, but they won’t explain the idea to someone who genuinely lacks the ability to discriminate the good from the bad, or help us better understand the nature of good argumentation so we can improve our own ability.

We can sneak up on the problem by focusing on the bad argument. It has a hole in it: it could be that all Xs have lungs, but there are other species that also have lungs, and so, y could be one of those. If Xs are dogs, and cats also have lungs, then maybe y is a cat. So, the conclusion would be false.

As it happens, lots of species have lungs. But even if there weren’t any other species with lungs, the premises leave open the possibility that such species exist and that is enough for the argument to have a hole in it. And this hole is what makes it a bad argument. The first argument is good because it has no hole; it’s airtight: if all type Xs have lungs, and y is an X, the conclusion that y has lungs is inescapable.

There’s something to this, but unfortunately, talk of holes is just a metaphor here, and so, it’s too wooly to provide precise guidance. There’s not literally a hole in the argument, as when we say there’s a hole in the wall or in my sweater. For someone who doesn’t already have the skill of evaluating arguments, telling them to look for holes is vague, hand-waving advice.

But it does capture something important. So, we need to figure out the precise idea to which the metaphor points. Here’s a way of putting it: what is special about the first argument is that the truth of the premises would absolutely guarantee the truth of the conclusion; if the premises were true, the conclusion would have to be true. Or, to put it most precisely: it is impossible for both the premises to be true and the conclusion to be false together. This statement is non-metaphorical, and it explains the goodness of the argument in simple terms that do not presuppose specialized logical knowledge: impossible, true, and false. We call arguments like this “valid.”

An argument is valid if and only if it is impossible for the premises to be true and the conclusion to be false together.

The other ones, the ones that lack this special property, we call “invalid.”

This definition fits with our two examples. What makes the first argument good is that it is absolutely impossible for it to be false that y has lungs, given that it is true that y is an X and all Xs have lungs. What makes the second example bad is that it clearly is possible for all Xs to have lungs, and for y to have lungs, and yet for y not to be an X (i.e., for it to be false that y is an X). And if we fail to recognize the disconnect between the second argument’s premises and its conclusion, we are clearly allowing ourselves to be misled, to be persuaded by premises that just don’t provide a good reason to believe the conclusion. On the other hand, if we allow ourselves to believe the conclusion of the first argument, given its premises, we make no such error.

More generally, in life we typically want to believe truths and only truths. To have any success at that goal, we need to have some kind of policy for deciding what to believe. Here’s one policy: every time you are confronted with a proposition, flip a coin. If the coin comes up heads, undertake to believe the proposition; if it comes up tails, don’t. This is an obviously bad policy. If you followed it, any truths you came to believe would be a matter of sheer luck, and if you acted on the beliefs you acquired, you probably wouldn’t do very well. “Eating the rat poison will be a nutritious and delicious experience”: Let’s flip a coin.

We need a policy that tracks the truth: picks out truths and avoids falsehoods. Picking out valid arguments and rejecting invalid ones is part of such a policy, a crucial component of it. However, just paying attention to validity is not enough. Validity on its own provides no reason to believe the conclusion is true. And this is made explicit in our definition: all it says is that a valid argument can’t have true premises and a false conclusion. It guarantees conditional support between the premises and the conclusion: If the premises are true, then the conclusion must be too. If not, all bets are off. Valid arguments with false conclusions are not hard to find. For instance:

• (P1) All human beings have tentacles.
• (P2) All creatures with tentacles live in the sea.
• (C) So, all human beings live in the sea.

It’s valid, but it provides no reason to believe the conclusion. Why? Because one of the premises is obviously false, and valid reasoning from a false premise provides no reason whatsoever to believe that we have a true conclusion.

Another way of putting this is to say that valid arguments are truth-preserving: all true premises guarantee a true conclusion: Truth in; truth out. Falsehood in; who knows? (Unless you’re lucky, a false conclusion.) Certainly, the argument gives you no reason to believe it true. So, our policy for truth-tracking should be this: believe only the conclusions of arguments that are valid and that have all true premises. These arguments are important enough that we need a name for them. We’ll say an argument is sound if and only if it is valid and has all true premises. A sound argument must have a true conclusion: Truth-preservation + all true premises guarantees a true conclusion.

A word of caution: these two features of an argument (i) its validity, and (ii) the actual truth values of its premises, have nothing to do with each other. It is worth emphasizing this point, as people often mistakenly think that the actual truth values of the premises and conclusion—whether the premises and conclusion happen to be true or false—can tell us whether the argument is valid or not. But this is not so. For example:

• (P1) Beethoven’s music is excellent.
• (P2) If someone’s music is still well-known centuries after their death, their music must be excellent.
• (C) So, Beethoven’s music is still well-known centuries after his death.

It’s perfectly possible that someone might have written excellent music and also that music only survives the test to time if it is truly excellent, and yet some great composer could be unlucky enough for their work to be lost or destroyed before achieving any popularity, and so never be well-known. That’s not how it was for Beethoven, but it could have happened. So, the argument is invalid, and yet the premises and conclusion are all plausibly true. Validity and true premises guarantee a true conclusion, but it doesn’t work the other way around: true premises and a true conclusion guarantee nothing about the quality of reasoning.

Just to hammer home the point, let’s return to our first pair of arguments:

• (P1) All members of species X have lungs.
• (P2) y is a member of species X.
• (C) Therefore, y has lungs.

• (P1) ...