CHAPTER 9

VALID AND SOUND ARGUMENTS

9.1 VALIDITY AND SOUNDNESS

Validity is a most important concept in critical thinking. A valid argument is one where the conclusion follows logically from the premises. But what does it mean? Here is the official definition:

An argument is valid if and only if there is no logically possible situation in which the premises are true and the conclusion is false.

To put it differently, whenever we have a valid argument, if the premises are all true, then the conclusion must also be true. What this implies is that if you use only valid arguments in your reasoning, as long as you start with true premises, you will never end up with a false conclusion. Here is an example of a valid argument:

This simple argument is obviously valid since it is impossible for the conclusion to be false when the premise is true. However, notice that the validity of the argument can be determined without knowing whether the premise and the conclusion are actually true or not. Validity is about the logical connection between the premises and the conclusion. We might not know how old Marilyn actually is, but it is clear the conclusion follows logically from the premise. The simple argument above will remain valid even if Marilyn is just a baby, in which case the premise and the conclusion are both false. Consider this argument also:

Again the argument is valid—if the ...