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. Top Next Issue
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