Logically Illogical

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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