Critical Thinking by Example

Chapter 2: Elementary Argument Evaluation

Material covered in this chapter 2.1 The definition of a good argument 2.2 Three Fallacies

2.1 The Definition of a Good Argument

A good argument is one that meets these three conditions: (1) The premise set is relevant to the conclusion, (2) the premise set is sufficient for the conclusion, and (3) the premises are acceptable.

Example 2.1: A good argument.

[1] All humans are mortal. [2] Socrates is a human. [3] So, Socrates is mortal.

The premises [1] and [2] are acceptable, since they are true. The premises are relevant to the conclusion [3] and sufficient, so this is a good argument.

2.2 Three Fallacies

It may help to understand the definition of a good argument by considering bad arguments, that is, arguments that fail to meet one of the three conditions for a good argument. We will define three fallacies, characteristic ways people make bad arguments, to illustrate failure of each of these conditions.

Fallacy of Irrelevant Reason Definition: An argument contains this fallacy if the premise set is irrelevant to the question of whether we should accept the conclusion. Note: example 2.2 is purposely ridiculous to exemplify the underlying pattern. Even though the premise is true, it provides us no reason to believe that the conclusion is true. The premise is irrelevant to the conclusion. Most arguments that commit the fallacy of irrelevant reason are more subtle than this, but the underlying pattern is the same.