A fallacy is an argument that may be persuasive but contains an error of logic or language.
A fallacy is an error in reasoning, but reasoning can be erroneous in a number of ways, so there is no definitive type of fallacy.
Aristotle wrote in On Sophistical Refutations (c. 350 BCE)
That some reasonings are genuine, while others seem to be so but are not, is evident. This happens with arguments, as also elsewhere, through a certain likeness between the genuine and the sham. For physically some people are in a vigorous condition, while others merely seem to be so by blowing and rigging themselves out as the tribesmen do their victims for sacrifice; and some people are beautiful thanks to their beauty, while others seem to be so, by dint of embellishing themselves. So it is, too, with inanimate things; for of these, too, some are really silver and others gold, while others are not and merely seem to be such to our sense; e.g. things made of litharge and tin seem to be of silver, while those made of yellow metal look golden. In the same way both reasoning and refutation are sometimes genuine, sometimes not, though inexperience may make them appear so: for inexperienced people obtain only, as it were, a distant view of these things. For reasoning rests on certain statements such that they involve necessarily the assertion of something other than what has been stated, through what has been stated: refutation is reasoning involving the contradictory of the given conclusion. Now some of them do not really achieve this, though they seem to do so for a number of reasons; and of these the most prolific and usual domain is the argument that turns upon names only. It is impossible in a discussion to bring in the actual things discussed: we use their names as symbols instead of them; and therefore we suppose that what follows in the names, follows in the things as well, just as people who calculate suppose in regard to their counters.
In the subsequent centuries of philosophical debate, new categories of fallacies were identified, and the philosophers William of Ockham and John Buridan compiled an extensive number of fallacy types, giving them Latin names such as argumentum ad populum (appeal to the people) and argumentum ad baculum (appeal to the stick, or force).
There are now more than 200 named fallacies, commonly divided between formal and informal.
- Formal fallacies are mistakes in the logical form of an argument, independent of its semantic content. For example, in the non-fallacious form called Modus Ponens, a correct deduction can be derived from a conditional premise and a correct antecedent, regardless of the content. However, in the related formal fallacy called “affirming the consequent,” a false deduction is derived from the same correct conditional premise and a false antecedent. It follows that not every instance of the deduction would be true, even if the premise statements appeared correct individually.
- An informal fallacy occurs when the content or organization of the premises of an argument constitutes an error in reasoning, as when an arguer changes the subject (red herring) or appeals to an inappropriate authority (argumentum ad verecundiam).