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An informal fallacy (also relevance fallacy, conceptual fallacy, soundness fallacy) is an argument that is formally valid but is unsound because of the falsity or irrelevance of one or more of its premises. Informal fallacies are often characterised by the fact that there is a disconnect of some kind between their premises and conclusions. Inappropriate generalisations and argument from ignorance are forms of informal fallacies. These tend to be more numerous and slightly more interesting than formal fallacies.
- P1: Half of all adult humans can grow beards.
- P2: Sophia is a woman.
- C1: There is a 50% chance Sophia can grow a beard.
This is not a sound argument because the first premise is irrelevant. Because Sophia is a woman, we know she is in the portion of the population that is generally incapable of growing beards. The fact that half of the population can grow beards merits no consideration since we already know which half Sophia belongs in.
In the "Destiny of the Daleks" episode of Doctor Who the following invalid syllogism is presented:
- P1: All elephants are pink.
- P2: Nellie is an elephant.
- C1: Nellie is pink.
The Informal fallacy here is the first statement that "All elephants are pink" but as a character in the story points out "Elephants aren't pink". So the syllogism is invalid because its first premise is wrong.
 See also
- Category:Informal fallacies
- Logical fallacy
- Formal fallacy
- Fallacious argument style
- Conditional fallacy