Refutation in Practical Reasoning
In sound reasoning the conclusion you infer must follow from the premises. If the reason is inductive, this means that its conclusion must follow with a degree of probability that makes it reasonable to believe (and act on) the conclusion. If the reasoning is deductive, the conclusion must necessarily follows from the premises. That is, the argument must be formally valid—have a valid form—and therefore avoid formal fallacies such as those discussed earlier in this book. However, sound reasoning requires more than just that the conclusion follow from the premises. The premises themselves must also be rational or justified. As you have already seen, the conclusions of valid deductive arguments can turn out false unless the premises are also true.
The importance of having rational premises in doing practical reasoning cannot be overstated. Recall how former CBS journalist Dan Rather and even the esteemed New
York Times betrayed the public trust by blindly accepting whatever government officials told them, and how President Bush led us to war in Iraq on unjustified and false premises
(see Chapter 13). Whether someone is making a decision that could affect thousands of people—as in the case of Rather, the New York Times and the President of the United
States—or a decision that could affect one’s own personal welfare, the avoidance of unjustified or false premises in practical reasoning can be of inestimable value.
Accordingly it is useful to have at one’s disposal some methods for showing a premise to be false or unjustified. Such methods may be called methods of refutation, and there are several kinds: (1) deductive falsification, (2) showing that there is
insufficient inductive evidence, (3) reductio ad absurdum, and (4) showing a double standard. In what follows we will examine each of these methods of refutation.
In some cases a premise can deductively be shown to be false by presenting evidence that contradicts the premise. For example, the use of DNA testing has been successfully used to refute the premises of legal reasoning that have sent at least 130 innocent people to prison, including 12 who spent time on death row.
One such individual was Ray Crones, who spent ten years of life in prison, three of which on death row, for the murder of a Phoenix cocktail waitress he did not commit.
Falsely branded the ―snaggletooth killer,‖ on the basis of bogus forensic evidence attempting to match his teeth to the teeth marks on the neck of the victim, he was later exonerated by DNA analysis of the saliva left on the tank top worn by the victim.
If Crones was the snaggletooth killer, then he was guilty of a capital crime. This was the gist of the practical rule that was applied in his case. But the crux of the legal reasoning that led the jury to conclude that Crones was, in fact, the snaggletooth killer can be cast as the following valid hypothetical syllogism:1
Major Premise: If Crone’s teeth caused the wound on the victim’s neck, then he killed the victim
Minor Premise: Crone’s teeth caused the wound on the victim’s neck
Conclusion: Crones killed the victim
Despite the fact that there was further evidence to disprove the above minor premise it was not until DNA testing turned up a match between the DNA in the saliva and another person who was already serving time in an Arizona prison for child molestation that
Crones was exculpated. The syllogism that falsified this premise was accordingly this one: Major Premise: If Crone’s teeth caused the teeth marks on the victim’s neck then the saliva on the areas of the victim’s tank top where Crones bit through it would match Crone’s DNA.
Minor Premise: The DNA on the tank top did not match Crone’s DNA
Conclusion: Crone’s teeth did not cause the teeth marks on the victim’s neck
As you can see, the syllogism is valid—a case of denying the consequent.