Irrelevant Reason

This fallacy is a kind of non sequitur in which the premises are wholly irrelevant to drawing the conclusion.

Example:

Lao Tze Beer is the top selling beer in Thailand. So, it will be the best beer for Canadians.

Invalid Reasoning

An invalid inference. An argument can be assessed by deductive standards to see if the conclusion would have to be true if the premises were to be true. If the argument cannot meet this standard, it is invalid. An argument is invalid only if it is not an instance of any valid argument form. The fallacy of invalid reasoning is a formal fallacy.

Example:

If it's raining, then there are clouds in the sky. It's not raining. Therefore, there are no clouds in the sky.

This invalid argument is an instance of denying the antecedent. Any invalid inference that is also inductively very weak is a non sequitur.

Affirming the Consequent

If you have enough evidence to affirm the consequent of a conditional and then suppose that as a result you have sufficient reason for affirming the antecedent, you commit the fallacy of affirming the consequent. This formal fallacy is often mistaken for modus ponens, which is a valid form of reasoning also using a conditional. A conditional is an if-then statement; the if-part is the antecedent, and the then-part is the consequent. The following argument affirms the consequent that she does speaks Portuguese.

Example:

If she's Brazilian, then she speaks Portuguese. Hey, she does speak Portuguese. So, she is Brazilian.

If the arguer believes or suggests that the premises definitely establish that she is Brazilian, then the arguer is committing the fallacy. See the non sequitur fallacy for more discussion of this point.