Fallacy

The noun fallacy – pronounced with the stress on the first syllable and a soft ‘c’, IPA: /'fæ lə sɪ/ - is used in two ways:

to denote a false belief, e.g., ‘It is a fallacy that sugar causes hyperactivity in children’ or ‘It is a fallacy that at least 24 hours must elapse before a missing person claim can be filed with the police’. Typically the belief characterised as a fallacy will be one which despite its falsity is widely held. Fallacy in this use may be replaced by misconception or common misconception.

to denote a logical error or mistake in reasoning. For example, if I were to argue that since all Italians like pizza and Laura likes pizza, Laura must be Italian, my reasoning would be faulty: I should have committed a fallacy, specifically, the Fallacy of the Undistributed Middle. Note that I have committed a fallacy, even if the conclusion of my reasoning is true and Laura is, as it happens, Italian: it is possible by faulty reasoning to reach a conclusion which happens to be true. (Many fallacies were first identified and named by the Greek philosopher Aristotle (382-322 BCE) – for more information about this and the various types of fallacy consult any standard logic textbook.)

The adjective from fallacy is fallacious – pronounced with the stress on the second syllable, IPA: / fə 'leɪ ʃəs/.

Etymological note: Fallacy comes from the Latin fallācia (’trick’, ‘deception’), a noun from the adjective fallax (‘deceitful’, ‘deceptive’), which in turn comes from the verb fallĕre (‘to deceive’). This Latin verb is also the root of the English adjective false, through the Latin falsus (‘false’, ‘deceitful’, ‘sham’, ‘fictitious’) and the Old English fals.