Fallacies are generally classified strictly by either their structure or their content, such as by classifying them as
formal fallacies or
informal fallacies, respectively. The classification of informal fallacies may be subdivided into categories such as linguistic, relevance through omission, relevance through intrusion, and relevance through presumption. Alternatively, fallacies may be classified by the process by which they occur, such as
material fallacies (content),
verbal fallacies (linguistic), and formal fallacies (error in inference). In turn, material fallacies may be placed into the more general category of informal fallacies. Verbal fallacies may be placed in either formal or informal classifications: Compare
equivocation, which is a word- or phrase-based
ambiguity, to the
fallacy of composition, which is premise- and inference-based ambiguity.
Greek logic The Greek philosopher
Aristotle (384–322 BC) was the first to systematize logical errors into a list to make it easier to refute an opponent's thesis and thus win an argument. Aristotle's
Sophistical Refutations () identifies thirteen fallacies. He divided them up into two major types: linguistic fallacies and non-linguistic fallacies, some of which depend on language and others that do not. These fallacies are called verbal fallacies and material fallacies, respectively. A
material fallacy is an error in what the arguer is talking about, while a
verbal fallacy is an error in how the arguer is talking. Verbal fallacies are those in which a conclusion is obtained by improper or ambiguous use of words. An example of a language dependent fallacy is given as a debate as to who in humanity are learners: the wise or the ignorant. A language-independent fallacy is, for example: • "
Coriscus is different from
Socrates." • "Socrates is a man." • "Therefore, Coriscus is different from a man."
Indian logic Indian logicians took great pains to identify fallacies in arguments. An influential collection of texts on logic and reason, the
Nyāya Sūtras, attributed to
Aksapada Gautama, variously estimated to have been composed between the 6th century BCE and the 2nd century CE, lists in its theory of inference five such reasons used in an argument that was further developed by later logicians. •
Asiddha: It is the unproved reason that results in this fallacy. [Paksadharmata] •
Savyabhichara: This is the fallacy of irregular reason. •
Satpratipaksa: Here the reason is contradicted by another reason. If both have equal force, then nothing follows. 'Sound is eternal, because it is audible', and 'Sound is non-eternal, because it is produced'. Here 'audible' is counterbalanced by 'produced' and both are of equal force. •
Badhita: When another proof (as by perception) definitely contradicts and disproves the middle term (reason). 'Fire is cold because it is a substance'. •
Viruddha: Instead of proving something it is proving the opposite. 'Sound is eternal because it is produced'.
Whately's grouping English scholar and theologian
Richard Whately (1787–1863) defines a fallacy broadly as, "any argument, or apparent argument, which professes to be decisive of the matter at hand, while in reality it is not". Whately divided fallacies into two groups:
logical and
material. According to Whately, logical fallacies are arguments where the conclusion does not follow from the premises. Material fallacies are not logical errors because the conclusion follows from the premises. He then divided the logical group into two groups: purely logical and semi-logical. The semi-logical group included all of Aristotle's
sophisms except
ignoratio elenchi,
petitio principii, and
non causa pro causa, which are in the material group.
Other systems of classification Other famous methods of classifying fallacies are those of
Francis Bacon and
J. S. Mill. Bacon (
Novum Organum, Aph. 33, 38 sqq.) divided fallacies into four Idola (Idols, i.e. False Appearances), which summarize the various kinds of mistakes to which the human intellect is prone. J. S. Mill discussed the subject in book five of his Logic, and
Jeremy Bentham's
Book of Fallacies (1824) contains valuable remarks. == Formal fallacy ==