Point Lookout: a free weekly publication of Chaco Canyon Consulting
Volume 9, Issue 41;   October 14, 2009: Logically Illogical

Logically Illogical

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Discussions in meetings and in written media can get long and complex. When a chain of reasoning gets long enough, we sometimes make fundamental errors of logic, especially when we're under time pressure. Here are just a few.
Well-wishers greet physicist Stephen Hawking (in wheelchair) at the Kennedy Space Center Shuttle Landing Facility

Well-wishers greet physicist Stephen Hawking (in wheelchair) at the Kennedy Space Center Shuttle Landing Facility after a zero gravity flight in 2007. On July 31, 2009, Investor's Business Daily ran an editorial warning that adoption of a particular proposal for healthcare reform in the United States would lead to chaos. It read, in part, "People such as scientist Stephen Hawking wouldn't have a chance in the U.K., where the National Health Service would say the life of this brilliant man, because of his physical handicaps, is essentially worthless." This argument is fallacious on multiple grounds, but many debunkers focused on the fact that Professor Hawking has received almost all of his medical care from the UK National Health Service. As Hawking told the Guardian, "I wouldn't be here today if it were not for the NHS."

If participants in the current debate about US healthcare reform were to use this incident as evidence in favor of their proposals, they might be committing the error here called the sand castle fallacy, because they would be rejecting the conclusion, put forth by Investor's Business Daily, that the proposals are unsound, on the basis that the IBD editorial included a false premise. The reform proposals might be sound or unsound, but one cannot determine which on the basis of this unsound editorial. However, the incident might indeed serve as part of a case that the editorial opinions of Investor's Business Daily ought to be taken with more than a grain of salt. Photo credit: NASA/Kim Shiflett.

Faulty logic is one reason why groups make defective decisions. In long discussions, spanning many meetings, email messages and other postings, a chain of logic emerges. Usually, the chain is valid and coherent, but when the conversation becomes complex, when the stakes are high, or when time is short, a group can make logical errors. Here's a small collection.

The fallacy fallacy
If the argument used to reach our conclusions is later found to be flawed, we sometimes believe that the conclusion produced by that argument must also be false.
Not so. That we reached a particular conclusion by faulty logic doesn't mean that the conclusion itself is false. The conclusion might be true, or it might be false.
Sand castle fallacy
If the argument is founded on premises that later prove to be false, or partly false, we sometimes believe that the conclusion produced by that argument must also be false.
Not so. That we reached a particular conclusion based on faulty premises doesn't mean that the conclusion is false. We might have built a castle on sand, but it might still be a castle. If we can find a better foundation for that castle, it might yet prove durable.
Affirming a disjunct
If we know that A or B is true, and it turns out that A is true, we sometimes conclude that B must be false.
The fallacy here arises in instances when A and B are both true. In informal conversation, we often use "or" in the exclusive sense: either A or B, but not both. But in many situations, "or" actually is valid in the inclusive sense: A or B or possibly both A and B.
Affirming the consequent
When a conversation becomes
complex, when the stakes
are high, or when time
is short, a group can
make logical errors
In this error, sometimes called the converse error, we conclude incorrectly that a premise must be true if the conclusion is true. That is, when we know that P implies Q, and we know that Q holds, we wrongly conclude that P must also be true.
The problem here is that the converse of a true statement isn't necessarily true. The contrapositive is true, though: if not Q then not P.
Denying the antecedent
This formal fallacy, sometimes also called the inverse error, is committed when we know that if P, then Q. If we later find that P is false, we then sometimes conclude erroneously that Q is false.
All we can say for sure is that Q might be false when P is false; Q might also be true.

Keeping these errors and their names straight can be difficult, but learning to recognize and avoid them is certainly easier. There is a trap, though. Once you notice that a group has committed one of these errors, remember that it's often possible that their conclusion is correct. To forget that possibility is to commit the fallacy fallacy. Go to top Top  Next issue: The Attributes of Political Opportunity: The Basics  Next Issue

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