The Fallacy of Negative Premises and 5 Cases that Failed

Why is this wrong?  

Laypersons are not competent to testify as expert witnesses.

Mr. Flashburger is a layperson.

Therefore, Mr. Flashburger is competent to testify as a fact witness.*           

We can intuitively recognise that the above argument makes no sense. However, can you explicitly identify the logical error?

The logical fallacy committed is named the Fallacy of Negative Premises.

Professor Stephen M. Rice has written a great paper on this Fallacy and provides us with a series real case examples.

What is the Fallacy of Negative Premises?

Put simply, the fallacy of Negative Premises is committed when one arrives at a positive conclusion, with a negative premise.

Let’s look at the example above.

 

The fallacy was committed because the argument used a negative premise (‘not competent’) and a positive conclusion (‘is competent’).

This seems obvious when broken down into raw components, however, it can be tricky to detect the fallacy amongst a complicated argument.

5 Case Examples

1. State v Weber

In the case State v Weber, a police officer pulled over Weber for driving over the speed limit. At the same time, the officer tested the driver’s alcohol level and found he was in-excess. However, Weber attempted to suppress this fact. He argued the police wanted to pull him over because of his speeding, not because of his alcohol levels. Although, there was no evidence of the officer’s motivations.

The argument can be broken down as follows:

Major premise: the police pulled over the driver;

Minor premise: there is no evidence of the police’s motivation;

Conclusion: Therefore, the police pulled over the driver motivated by something other than enforcing a speeding law.

The court detected the fallacy and stated, ‘it is a long-recognized logical fallacy to draw an affirmative conclusion from a negative premise.’

The argument was fallacious because it contained a negative premise (‘no evidence of motivation’) and a positive conclusion (‘motivated by’).    

2. State v Lackey

In this case, Lackey was accused of rape and murder. He defended himself by arguing that further DNA testing should be conducted.

The argument failed.                     

Major premise: Not all DNA samples were tested;

Minor premise: Hairs found on the victim's body were not tested;

Conclusion: Therefore, the hairs found on the victim's body should be tested.

This example is a tough one because we can intuitively understand the point that Lackey is trying to make.

After recognising the fallacy, Lackey corrected the argument but the court rejected it anyway.

Again, the argument was fallacious because it contained two negative premises (‘Not all DNA’ and ‘Hair’s… were not tested’) and an affirmative conclusion (‘should be tested’).

3. Ochsner v. Idealife Insurance Company

In this case, Ochsner tried to obtain insurance from ‘Idealife’ however, the insurance premium had not been paid for 9 years.

Ochsner won the case due to a logical technicality.                         

The legislation reads:                    

No life insurer shall within one year after default in payment of any premium, . . . declare forfeited or lapsed any policy . . . [without giving statutorily prescribed notice].

Idealife argued ‘that it provided notice of cancellation to’ Ochsner’s bank, but not to the Ochsner herself. However, ‘one cannot reach any affirmative conclusion (All life insurance policies unpaid after a year must lapse)’ with a negative premise (did not give notice to the insured).’                    

4. Walmsley v. City of Philadelphia

The Facts:                                    

An expert witness in a police brutality case testified that a man died from multiple blows to the head. The plaintiff argued that the blows occurred as a result of police brutality, while the police claimed that the man died from blows received in a fight that occurred prior to any police confrontation.

The parties agreed that prior to entering police custody, the deceased had been in a fight with his brother, and that near the end of the fight the deceased was struck two or three times in the face. Sometime after the fight, police arrived and took the deceased into custody. While the police were transporting him to the police station, the deceased lost consciousness and died.

The deceased's family claimed that before the police confrontation, they had an opportunity to observe the deceased, brushed his hair back from his face, and saw no lumps on his head. Afterwards, however, lumps were visible. The plaintiff argued that since the witnesses observed no lumps before the police confrontation, but observed them after, the lumps must not have been caused by the fight that preceded the confrontation.

However, the judge retorted and did an exceptional job in explaining the fallacy:     

This type of reasoning is unacceptable because of the difficulty in sustaining a factual proposition merely by negative evidence.

When an advocate determines that there is no evidence that B (bumps on the head) is the case; he or she is attempting to affirm or assume that non-B is the case. But all that is affirmed or assumed is that the advocate has found no evidence of non-B. The correct method of proceeding is to find affirmative evidence of non-B.

This may be difficult, but it is absolutely necessary if logical order is to be preserved. To prove a negative is sometimes an impossible task. Not knowing that something exists is simply not knowing.

5. Kolakowski v. Secretary of Health and Human Services                                

Kolakowski argued ‘that their son had died as a result of the Thimerosal contained in two Hepatitis B vaccinations.’ Kolakowki filed a petition to get compensation and their argument can be reduced  to the following syllogism:

 

Again, we can see that Kolakowski committed the Fallacy of Negative Premises.

Prescriptions

  1. Watch out for the Fallacy of Negative Premises.
     
  2. Look for a negative premise, then a positive conclusion. Judge Green sums it up best, 'If one premise is negative, the conclusion must be negative.' If not, The Fallacy of Negative Premises is committed.