Chapter One, Tutorial Three
Evaluating Arguments

We now have some experience with how people express their reasoning. But you will have noticed that arguments vary greatly in quality. Some of them seem quite good and convincing. Some arguments are silly and/or lack force. It's time to think about evaluating arguments.

To reiterate, we have defined an argument this way:

An argument is a collection of statements including some (the premises) that are given as reasons for another (a conclusion).

But if premises are given as reasons, then that means someone intends that they support the conclusion. An argument is successful, then, when it does support its conclusion, i.e., when it shows the conclusion is true or likely true.

In an argument, the premises are supposed or intended to support the conclusion, to show it true or likely true.

To evaluate an argument, then, we need to check that its premises really do count as good evidence for its conclusion.

Before we develop clear definitions for evaluating formal/deductive and informal/inductive arguments, let's look to some cases that will help with intuitions. Be patient as we slowly motivate the definitions to come.

We began this chapter with an example of an argument for which the premises (if true) are sufficient evidence to prove their conclusion:

Chris will get an 'A' or a 'B' in logic this term. But, he will not receive an 'A'. Thus, he will receive a 'B'.

In a moment, when we give definitions, we will call this argument about Chris valid. Roughly, the idea behind a valid argument is that the premises provide evidence making the conclusion inescapable. In this case, the conclusion is inescapable given the truth of the premises because of the form we've called DS.

DS is just process-of-elimination thinking; any argument of it's form is valid. This notion of a valid argument, then, is a part of formal, deductive logic.  But...

Poor Arguments

Before giving the definitions, it's worth looking at some arguments that are not so good.

The 2003 invasion of Iraq was wrong. Bush is a draft dodger, an illegitimate president, and we should not have followed such a man into battle.     (Problem: we will call the argument "weak" because the premises in and of themselves aren't sufficient to substantiate the conclusion.)

This thinking makes an unfortunate mistake: an attack on Bush is used as a reason against a war. Although there is a connection between the advisability of a war and a president's character, an attack on Bush's past is no substitute for reasoning about the war itself.

The problem here is sometimes called an ad hominem fallacy: the reasoning is poor or fallacious because it's an irrelevant attack on a proponent of a cause rather than reasons that address the issue at hand. So, the content, the ideas expressed against Bush, do not themselves make a particular strategy the wrong one. The premises themselves make the conclusion neither inescapable nor likely to be true. If you want to argue that the 2003 invasion of Iraq was a terrible mistake, then you need to do more than sneer at Bush.

We don't yet have all the clear definitions, but the idea will be that this argument is informal/inductive but not strong. The problem with this argument is that the content or ideas expressed by the premises are not relevant to the conclusion.

Moving on...

We've just seen an example of poor inductive reasoning. But sometimes the fault of an argument will lie not with its content (what is said) as with its form (its structure). Consider:

Sanchez stays at her banking job only if she gets a raise. So, if she gets a raise, she'll continue at the bank.   (Problem?)

This reasoning may at first seem OK. But it's not. To begin to see the problem, notice that the same form of reasoning is obviously wrong in a different context:

There is fire only if there's oxygen. So, if we add oxygen to an area, there will be fire.    (This has the same form and the same problem as the Sanchez argument!)

To conclude there will be fire in an area with oxygen is to make a mistake. Often we have oxygen but no fire: oxygen is a requirement for burning but is not sufficient in itself. One also needs fuel and a spark.

Similarly for Sanchez: her staying on the job may require that she get a raise, but she may have other requirements as well. (Say, she's pregnant and needs special incentives to stay on the job: reduced office time, a means to work from home, child care. Additional money may be only one requirement of many.) The bottom line for the Sanchez argument, just like the fire-oxygen argument, is that the one premise is not sufficient for the conclusion.

The problem with this Sanchez case is a problem of poor form. Drawing the conclusion that she'll stay because the one mentioned requirement is met is ignoring the possibility of other requirements. Any reasoning of that form is incorrect (or "fallacious".)

Here's an easier example of poor form that we've already seen:

Definitions for Deductive Arguments

We need to say more about good and bad arguments. Let's begin by thinking about formal, deductive logic and define "valid" argument. Roughly...