PHIL102: Introduction to Critical Thinking and Logic

§1. What is a good argument?

In this tutorial we shall discuss what a good argument is. The concept of a good argument is of course quite vague. So what we are trying to do here is to give it a somewhat more precise definition. To begin with, make sure that you know what a sound argument is.

§2. Summary

So, here is our final definition of a good argument :

A good argument is an argument that is either valid or strong, and with plausible premises that are true, do not beg the question, and are relevant to the conclusion.

Now that you know what a good argument is, you should be able to explain why these claims are mistaken. Many people who are not good at critical thinking often make these mistakes :

• "The conclusion of this argument is true, so some or all the premises are true."
• "One or more premises of this argument are false, so the conclusion is false."
• "Since the conclusion of the argument is false, all its premises are false."
• "The conclusion of this argument does not follow from the premises. So it must be false."

§3. A technical discussion

This section is a more abstract and difficult. You can skip this if you want.

One interesting but somewhat difficult issue about the definition of a good argument concerns the first requirement that a good argument must have true premises. One might argue that this requirement is too stringent, because we seem to accept many arguments as good arguments, even if we are not completely certain that the premises are true. Or perhaps we had good reasons for the premises, even if it turns out later that we were wrong.

As an example, suppose your friend told you that she is going camping for the whole weekend. She is a trustworthy friend and you have no reason to doubt her. So you accept the following argument as a good argument:

Amie will be camping this weekend. So she will not be able to come to my party.

But suppose the camping trip got cancelled at the last minute, and so Amie came to the party after all. What then should we say about the argument here? Was it a good argument? Surely you were justified in believing the premise, and so someone might argue that it is wrong to require that a good argument must have true premises. It is enough if the premises are highly justified (of course the other conditions must be satisfied as well.)

If we take this position, this implies that when we discover that the camping trip has been cancelled, we are no longer justified in believing the premise, and so at that point the argument ceases to be a good argument.

Here we prefer a different way of describing the situation. We want to say that although in the beginning we had good reasons to think that the argument is a good one, later on we discover that it wasn't a good argument to begin with. In other words, the argument doesn't change from being a good argument to a bad argument. It is just that we change our mind about whether the argument is a good one in light of new information. We think there are are reasons for preferring this way of describing the situation, and it is quite a natural way of speaking.

So there are actually two ways to use the term "good argument". We have adopted one usage here and it is fine if you want to use it differently. We think the ordinary meaning of the term is not precise enough to dictate a particular usage. What is important is to know very clearly how you are using it and what the consequences are as a result.