Many people are confused about what counts as a fallacy, including teachers of critical reasoning. Opponents of science often accuse pro-science writers of “the fallacy of authority” or “the ad hominem fallacy” when they are noted for having made silly and false claims before. I thought some words about what a fallacy actually is might be to the point.
By a Fallacy is commonly understood, “any unsound mode of arguing, which appears to demand our conviction, and to be decisive of the question in hand, when in fairness it is not.”
Basically, a fallacy is something designed to lead the hearer or reader to draw the wrong conclusion.
But because we have constantly to appeal to the conclusion of a previous process of demonstration or other reasoning without rethinking that process at the time, we are constantly syllogizing ; and where the premisses are such of which we remember to have previously satisfied ourselves by reflection or demonstration or inductive argument, or (if they concern facts established by authority) by reference to authority, there syllogism may deserve the name of proof. [p311]
The schoolmen knew, as well as Bacon or any other of their critics, that the study of the syllogism was not all-sufficing: that no syllogism could guarantee the truth of its premisses; and that for a knowledge of the most general principles to which deductive reasoning appeals we must rely on something else than deductive reasoning itself. Bacon refers to the ‘notorious answer’ which was given to those who questioned the accepted principles of any science — Cuique in sua arte credendum.* And there are seasons in the process of learning when that is a very proper answer; men must be content at many times and in many matters to accept the expert opinion of their day. But this is only tolerable if in every science there are experts who are for ever questioning and testing. When tradition stereotypes doctrine, it is as bad for knowledge as close guilds and monopolies are bad for the industrial arts; they shut the door upon improvement. Authority plays, and must play, a great part in life — not only in practice, but also in things of the intellect. But the free spirit is as necessary, which insists on satisfying itself that what is offered upon authority has claims on its own account upon our acceptance. [p376]
Appeal to authority is not fallacious, so long as the authority cited is relevant and reliable. A principle known as the division of cognitive labor (I think due to Hilary Putnam) suggests that we literally must rely on authorities in the absence of time, resources and cognitive capacities to rerun all experiments and observations since the beginnings of science and history. What is a fallacy is appeal to authority to make the listener conclude things that do not follow, either because the authority is unreliable or inappropriate, or because the authority is now known to be wrong (e.g. the constant quote mining of older biologists by creationists).
Likewise, ad hominem (the use of insult) is not a fallacy unless it is designed to mislead the listener. There is nothing wrong with calling someone who is a thief a thief. But if you do this in order to mislead a jury about his testimony against another criminal, that is a fallacy. Testing his testimony is something one does with evidence and reasoning. If he is a habitual liar, it is ad hominem (“to the man”) to bring that up; it is not fallacious, because that really does call his testimony into question.
Many things are said about fallacies on the internet these days which show a complete ignorance of the underlying principles of reasoning on which they are based. In my opinion, a fallacy is exactly what Whately said it was back in 1827 or so: something which demands our conviction which does not establish it. This definition goes back at least to Locke’s discussion of fallacies in Book IV, chapter XVII of his Essay.
There is no canonical list of fallacies because there are an indefinite number of ways to be wrong, but the division is conventionally made into fallacies of form and fallacies of rhetoric. Fallacies of form are mistakes in actual logic; rather like mistakes of arithmetic or mathematics. Fallacies of rhetoric are fallacies of how ideas are “sold”, such as the use of statistics or mathematics to mislead rather than elucidate. Politicians, when faced with data they do not like, usually engage in fallacies of rhetoric; however, they also make egregious mistakes of logical reasoning (the most common being the fallacy of affirming the consequent, in my opinion).
Go download Joseph and read bits of it at night. It is well worth it. Yes, there are modern texts you can get, but that all lack that magisterial tone that really commands attention (oops, I think that might be a fallacy).
- Stephen Downes’ Guide to the Logical Fallacies
- Infidels': Logic and Fallacies
- Informal Fallacies
- The Fallacy Files
*?Each person is to be believed with regard to his own specialty?, from Bacon’s The Advancement of Learning, Book II. This is a legal principle as well.