Critical Thinking by Example

Chapter 9: Accepting Premises: The Question of Evidence

Material covered in this chapter 9.1 Premises based on common knowledge of the target audience. 9.2 Premises based on experience 9.3 Premises based on epistemic authority

We have seen one line of support for premises. In serial arguments a subconclusion works as a premise for a further conclusion, while the subconclusion itself has a subpremise set offered in support of it. It is clear that this cannot be a general model for accepting all premises. For if every premise itself requires a further premise in support, then either this process will go on forever, e.g., P1 requires P2 for support, and P2 requires P3 for support, and so on; or we will end up repeating ourselves (arguing in a circle). A good argument must have at least one “bottom level” premise—a premise that is not supported by further premises. Bottom level premises can be either main premises that are not supported by further premises, or subpremises that are not supported by further premises. Our question then is: When should we accept bottom level premises? We will examine three instances: common knowledge, experience and appeals to experts.

9.1 Premises Based on Common Knowledge of the Target Audience

For the most part, we have been thinking about evaluating the arguments of others. In thinking about when premises are acceptable it will help to turn the exercise around: let us think about constructing good arguments with acceptable bottom level premises. Consider these examples:

Examples 9.1-9.7: Candidate premises based on common knowledge.

9.1 The earth is moving thousands of miles per hour. 9.2 The earth is billions of years old. 9.3 Government officials should not steal public money. 9.4 A fetus deserves the same moral consideration as an adult person. 9.5 Marijuana should be legal. 9.6 The earth is the center of the universe. 9.7 There is a new pizza place in town.

Imagine the target audience is a typical university undergraduate class and you are asked to debate some topic. Premises 9.1-9.3 would be appropriate bottom level premises that would be accepted on the basis of common knowledge. Premises that are common knowledge do not require any additional support.

On the other hand, premises 9.4-9.7 will require some other form of support. There is significant disagreement about the moral status of the fetus, that is, its moral status is not a matter of common knowledge in the typical undergraduate class. The same is true of 9.5. Example 9.6 is an extraordinary claim that would take some very extraordinary evidence to convince people: you are not likely to find a single person in a university class who thinks 9.6 is true. Finally, 9.7 is probably not a matter of common knowledge: the fact that the restaurant is new probably means the existence of the pizza joint is not a matter of common knowledge.
The qualification that common knowledge is based on the target audience should not be overlooked. In societies with little contact with science and advanced technology, 9.1 and 9.2 may require additional support, that is, they could not be assumed to be common knowledge. On the other hand, these same cultures may accept 9.6. In a meeting of the amoralist club, 9.3 may not be common knowledge and require further support. Similarly, if you were giving an argument to the marijuana legalization club, you could safely assume that 9.5 was common knowledge. It would probably be safe to assume that 9.4 is common knowledge if you were presenting an argument at a Catholic Bishop’s convention.
It should be emphasized that just because some of these premises would not be accepted as common knowledge, does not mean that they could not be premises in an argument, and even a bottom level premise. 9.7, as we shall see, might be an acceptable bottom level premise if based on experience. A controversial premise like 9.4 or 9.5 will probably require additional arguments to be acceptable, so they are not likely to be accepted as bottom level premises in many contexts.