Affirming the Consequent

One of several valid forms of argument is known as modus ponens (the mode of affirming by affirming) and takes the following form: If A then C, A; hence C. More formally:

A ⇒ C, A ⊢ C.

Here, we have three propositions: two premisses and a conclusion. A is called the antecedent and C the consequent. For example, If water is boiling at sea level, then its temperature is at least 100°C. This glass of water is boiling at sea level; hence its temperature is at least 100°C. Such an argument is valid in addition to being sound.

Affirming the consequent is a formal fallacy that takes the following form: If A then C, C; hence A.

The error it makes is in assuming that if the consequent is true, then the antecedent must also be true, which in reality need not be the case. For example, People who go to university are more successful in life. John is successful; hence he must have gone to university. Clearly, John's success could be a result of schooling, but it could also be a result of his upbringing, or perhaps his eagerness to overcome difficult circumstances. More generally, one cannot say that because schooling implies success, that if one is successful, then one must have received schooling.

Argument From Consequences

Arguing from consequences is speaking for or against the truth of a statement by appealing to the consequences of accepting or rejecting it. Just because a proposition leads to some unfavorable result does not mean that it is false. Similarly, just because a proposition has good consequences does not all of a sudden make it true. As David Hackett Fischer puts it, “it does not follow, that a quality which attaches to an effect is transferable to the cause.”

In the case of good consequences, an argument may appeal to an audience's hopes, which at times take the form of wishful thinking. In the case of bad consequences, such an argument may instead appeal to an audience's fears. For example, take Dostoevsky's line, “If God does not exist, then everything is permitted.” Discussions of objective morality aside, the appeal to the apparent grim consequences of a purely materialistic world says nothing about whether or not the antecedent is true.

One should keep in mind that such arguments are fallacious only when they deal with propositions with objective truth values, and not when they deal with decisions or policies [Curtis], such as a politician opposing the raising of taxes for fear that it will adversely impact the lives of constituents, for example.

Straw Man

Intentionally caricaturing a person's argument with the aim of attacking the caricature rather than the actual argument is what is meant by “putting up a straw man.” Misrepresenting, misquoting, misconstruing and oversimplifying are all means by which one commits this fallacy. A straw man argument is usually one that is more absurd than the actual argument, making it an easier target to attack and possibly luring a person towards defending the more ridiculous argument rather than the original one.

For example, My opponent is trying to convince you that we evolved from monkeys who were swinging from trees; a truly ludicrous claim. This is clearly a misrepresentation of what evolutionary biology claims, which is the idea that humans and chimpanzees shared a common ancestor several million years ago. Misrepresenting the idea is much easier than refuting the evidence for it.

Genetic Fallacy

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Emotional Appeal

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Guilt by Association

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Appeal to the Bandwagon

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