Lecture Notes.

Module Eleven: Session Six

Formal Fallacies

An argument is fallacious when the reasons offered fail to warrant acceptance of the conclusion. The fallacies in this section are called formal fallacies because problems with the FORM or structure of the argument mean that the conclusion is not certain. In other words, whenever you encounter an argument using one of these forms, there is cause to doubt the conclusion.

Affirming the Consequent

If P, then Q. Q. Therefore, P.

The symbolic form looks like this;


If it is raining, then it is cloudy. It is cloudy. Therefore, it is raining.

Recall that whenever the antecedent is present, the consequent necessarily follows. But it doesn't work the other way around. The consequent's presence tells us nothing about the antecedent's presence. Experience tells you that there are times when it is cloudy (the consequent), but it is NOT raining. This shows that it is possible for the premises of this type of argument to be true and the conclusion to be false. Hence, it is not a valid argument form.

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