This fallacy may at first glance seem counter-intuitive; but it illustrates an important distinction between the validity of an argument and the truth of its premises or conclusions.
Given that by definition, the conclusion of an invalid argument does not logically follow from its premises, the conclusion of such an argument can be true whether or not its premises are true.
The Argument from Fallacy is the fallacy of analysing an argument and inferring that, since it is logically invalid or is otherwise fallacious, the conclusion of the argument must therefore be false. This fallacy is also called the ‘Fallacy Fallacy’ or argument to logic (argumentum ad logicam).
The general form of of this argument is:
Premise 1: If P, then Q.
Premise 2: P is a fallacious argument.
Conclusion: Therefore, Q is false.
This is a special case of denying the antecedent where the antecedent, rather than being a proposition that is false, is an entire argument that is fallacious. A fallacious argument, just as with a false antecedent, can still have a consequent that happens to be true. The fallacy is in concluding the consequent of a fallacious argument has to be false. To give an example:
John: “All dogs are animals. Scottie is an animal. This means Scottie is a dog”.
Betty: “Ah, you just committed the affirming the consequent fallacy. Sorry, you are wrong, which means that Scottie is not a dog”.
In this example, John has committed the affirming the consequent fallacy, but it does not logically follow that his conclusion that Scottie is a dog is false – it just means that his argument for saying so is invalid.
Another example might be a belief that because somebody is unable to defend a position well, then that position must be false. All that has really been demonstrated is that the person in question cannot adequately defend their position, which could happen to be true. On the other hand, if this person is relying on a fallacious argument to support their position, then we should be skeptical of that position unless and until it can otherwise be shown through evidence and valid argument to be true.