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A formal fallacy, deductive fallacy, logical fallacy or non sequitur (Latin for "it does not follow") is a flaw in the structure of a deductive argument that renders the argument invalid. The flaw can be expressed in the standard system of logic. [1] Such an argument is always considered to be wrong.
People often have difficulty applying the rules of logic. For example, a person may say the following syllogism is valid, when in fact it is not: All birds have beaks. That creature has a beak. Therefore, that creature is a bird. "That creature" may well be a bird, but the conclusion does not follow from the premises.
Non sequitur fallacy – where the conclusion does not logically follow the premise. [8] Masked-man fallacy (illicit substitution of identicals) – the substitution of identical designators in a true statement can lead to a false one. [9]
A non sequitur (English: / n ɒ n ˈ s ɛ k w ɪ t ər / non SEK-wit-ər, Classical Latin: [noːn ˈsɛkᶣɪtʊr]; "[it] does not follow") is a conversational literary device, often used for comedic purposes.
A rule of inference is valid if, when applied to true premises, the conclusion cannot be false. A particular argument is valid if it follows a valid rule of inference. Deductive arguments that do not follow a valid rule of inference are called formal fallacies: the truth of their premises does not ensure the truth of their conclusion. [18] [14]
This differs from the fallacy known as post hoc ergo propter hoc ("after this, therefore because of this"), in which an event following another is seen as a necessary consequence of the former event, and from conflation, the errant merging of two events, ideas, databases, etc., into one.
The fallacy does not take into account whether the arguments do or do not really support the substituted issue, it only calls attention to the fact that they do not constitute proof of the original one… It is a particularly prevalent and subtle fallacy and it assumes a great variety of forms.
It is a type of mixed hypothetical syllogism that takes on the following form: [1] If P, then Q. Not P. Therefore, not Q. which may also be phrased as (P implies Q) (therefore, not-P implies not-Q) [1] Arguments of this form are invalid. Informally, this means that arguments of this form do not give good reason to establish their conclusions ...