How do you judge arguments? - Chapter 6

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Rational persuasion

In the previous chapters we have seen that an argument is correct if the premises are true and the argument is deductively valid or inductively powerful. However, this does not always apply; often we cannot say for certain whether the premises are true. View the following example:

P1) Interest rates will not fall in the coming year and will not remain the same

C) Interest rates will rise in the coming year.

This argument is deductively valid, but it will not convince anyone of the conclusion unless the person has already accepted it. We call such an argument rationally unconvincing. Whether the argument is rationally convincing depends on the extent to which a particular person has been informed at a given time (in this case how much a particular person knows about the interest rates).

The following example shows another complication that can occur:

P1) Almost all cats have a tail

P2) Rambo is a cat.

C) Rambo probably has a tail.

This argument is inductively powerful. But suppose you know from personal experience that Rambo's tail is amputated. Others would reasonably assume that C is true, but you know it is not true. We then say that the argument for you has been defeated by other evidence that you have. However, this can only occur with inductive arguments.

We can state that an argument for a person is rationally convincing if it is 1) either deductively valid or inductively powerful, 2) the person reasonably believes that the premises are true, and 3) if it is not an inductively powerful argument that is defeated for the person in question. Below are a number of points that are important for rational persuasion.

1. It is not possible that the conclusion of a deductively valid argument is defeated by the total evidence of a person. After all, the definition of a deductively valid argument is that if the premises are true, the conclusion is necessarily true. By using words such as "likely" for the conclusion of an inductively powerful argument, it can happen that the premises are true but the conclusion false.
2. It is possible that an argument is rationally convincing for a person, but that it contains a false premise and is therefore incorrect. There are two types of mistakes that people can make: I) a reasonable error (accepting a false conclusion) and II) an unreasonable error (being convinced by bad reasons). For example, if you know as a doctor about a certain medicine that it has always worked with a certain disease in the past, it is reasonable to believe that this person will also work with the same disease in person X. If person X turns out to be sicker because of the medication, this is a type I error.
3. Attempts at rational persuasion must be distinguished from attempts at persuasion that do not claim to reason (but, for example, to emotions or prejudices).
4. Rational persuasion is not a matter of all or nothing, but a matter of degree.
5. Whether an argument is rationally convincing does not depend on whether you think it is. Sometimes we are not convinced by rationally convincing arguments, or we are convinced by rationally unconvincing arguments. There are three ways in which we can be mistaken about the rational persuasiveness of an argument. We can make mistakes that are related to whether an argument is valid or inductively powerful. Secondly, we may think that we have a good reason to accept a premise when this is not the case. Thirdly, we can make mistakes as to whether or not an argument is defeated for you.
6. Assessments of rational persuasiveness often depend on the assessment of authority behind certain propositions. For example, a proposition from a newspaper is more likely to be seen as rationally convincing than a proposition from a gossip magazine.

Some strategies for logical assessment

To judge whether an argument is deductively valid, you may ask yourself whether there is a situation where the premises are true, but the conclusion is false. If this is not the case, the argument is valid. If that is the case you have to ask yourself which situations are more likely; those in which the conclusion is true or those in which the conclusion is false. In the first case you can say that the argument is, at least to a certain extent, inductive.

If the conclusion of an argument is a generalization or a conditional proposition, the above method is not always easy to use. We can use conditional evidence for this. You first use the conclusion as ... then convert form. Here you assume that the premises are true (P) and that the antecedent of the conclusion (Q) is true. If (P) together with (Q) is true, the conclusion's conclusion is also true, and the argument is valid. For example the argument:

P1) Every midfielder in the Italian team is a good defender

P2) Every player who tackles well is a good defender

C) Every midfielder in the Italian team tackles well

If we convert the conclusion, this becomes: If someone is a midfielder with the Italian team (Q), he tackles well. This makes it easier to see that this argument is not valid.

Another way to see if an argument is valid is to imagine that the conclusion is not true. If it is impossible for the premises to be false, then the argument is valid. If it is still possible for the premises to be true, the argument is invalid (as in the example above).

Rejection by means of a counterexample

Using a counterexample is a useful way to see if an argument is valid. For example, consider this argument:

P1) Almost all heroin addicts were cannabis smokers before they became heroin addicts

C) Cannabis smokers often become heroin addicts

To show that this argument is invalid, we make the assumption explicit: "If almost everyone who does X did Y for that, then those who did Y become people who do X".

If you reconstruct the argument with Y = drink milk, you immediately see that this argument is invalid:

P1) Almost all heroin addicts drank milk before they became heroin addicts

C) Milk drinkers often become heroin addicts

Argument criticism - Avoiding "who says that ..."

Criticism of an argument sometimes focuses on a premise of which no one can say with certainty whether it is true or not. For example, the argument: "smoking causes an increased risk of lung cancer" is a frequently heard criticism: "Who says I will get lung cancer? Maybe I will stay healthy until I am eighty! " This is empty criticism, because the critic gives no reason to believe that he / she is less likely to suffer from lung cancer than others. To effectively criticize an argument one must demonstrate that 1) the argument is invalid or inductive, or 2) demonstrate that there is a reason not to believe one of the premises, or 3) that it is an inductive argument that defeats. is by another argument.

Another point to avoid with argument criticism is trying to invalidate it by sticking a label on it (for example, "politically correct" or "biased").

Argument commenting

When analysing arguments (consisting of argument reconstruction and argument assessment) it is sometimes useful to write a summary of this process. This summary consists of 1) The argument as originally formulated, 2) The argument in standard form and 3) A comment on the argument. This last step must in any case include the following points:

1. A general discussion of the argument, such as the context and structure of the argument.
2. A discussion of how and why the standard form reconstruction was done in this way, including any problems associated with it.
3. A discussion of the validity or degree of inductive power of the argument
4. A discussion and assessment regarding the truthfulness of the premises
5. In the case of an inductively powerful argument, indicate whether the argument was defeated for you personally.

In addition, when preparing a comment you have to consider how detailed it should be; this can differ per argument. It is important that the reconstruction is explained and justified in an informative, clear, not-rhetorical and balanced manner (neutral).