There are two general types of fallacies in logic: formal and informal. Formal fallacies are errors in logic that are due entirely to the structure of the argument, it is an error in the process of building an argument - not the content of the premises, meaning that even if the premises are true, the argument cannot support its conclusion. Furthermore, this means that we can examine such arguments symbolically, without any reference to specific claims, so that we can know that no potential argument following this form can ever be valid. Aristotle held that the basis for all formal fallacies was the non sequitur, which is why the term is known in Latin as Ignoratio elench - or an ignorance of logic. For more on formal fallacies, see the Formal Fallacies section.
Informal fallacies have to do with the content of the premises. They occur when the "reasoning" or rationale behind the specific premise fails to support the conclusion. The support for the argument may instead rely on rhetoric (appeals to emotion) or another form of faulty logic.
These arguments also possess a particular form, so how do they differ from formal fallacies? They differ in that one cannot tell from the form alone that the argument is fallacious, because, in contrast to a formal fallacy, the error has to do with issues of inference manifest in language used to state the propositions. Because the range of elements that can be symbolized by language is broader than that which the symbolism of formal logic can represent, we must draw a distinction between the two. Formal fallacies of deductive reasoning contain a fundamental disconnect between the premises and the conclusion that renders the argument invalid. Informal fallacies also have a disconnection, but this disconnect stems from the presence of a hidden co-premise that, if presented, would validate the argument formally, but would in nearly all cases nullify the arguer's intention!
Below is a list of some common fallacies. The list isn't exhaustive; however, the hope is that if you learn to recognize some of the more common fallacies, you'll be able to avoid being fooled by them. This is not only of help in combating arguments, it's good for your psychological well being. According to Rational emotive behavioral therapy (REBT), the root cause of psychopathology is irrational beliefs. In other words, learning logic won't only help you win some arguments, but it may help you to live a healthier life in general! Cognitive and REBT therapists present their clients with a brief list of irrational thoughts. I present here a much more detailed list.
Types of informal fallacies[]
Other than the Self Refuting statement, there are six main types of informal fallacies. They are:
Fallacies of Ambiguity and Grammatical Analogy.
Internal Contradiction: Fallacies of Self Refutation[]
If you have read the section on the axioms of classic logic , you learned that classic Aristotelian logic is based on the law of non contradiction ¬ (P + ¬ P) , which in turn is defended through retortion - according to defenders of classical logic, denials of the law of non contradiction rely on the LNC. In the words of Aristotle, the law of non contradiction declares: "One cannot say of something that it is and that it is not in the same respect and at the same time (without falling to a contradiction)" And it follows from Aristotle's view of logic that all contradictions are false.
Therefore, the Self Refuting fallacy is the simplest fallacy to understand - it is a statement that renders itself false by containing within itself a(n) (internal) contradiction, and therefore, its own refutation. Because these argument trip over themselves, they are usually humorous in nature. Here is my all time favorite, from the biblical book of Numbers, supposedly written by Moses. I will let Thomas Paine explain it for you:
"But granting the grammatical right, that Moses might speak of himself in the third person, because any man might speak of himself in that manner, it cannot be admitted as a fact in those books, that it is Moses who speaks, without rendering Moses truly ridiculous and absurd: -- for example, Numbers xii. 3: "Now the man Moses was very MEEK, above all the men which were on the face of the earth." If Moses said this of himself, instead of being the meekest of men, he was one of the most vain and arrogant coxcombs; and the advocates for those books may now take which side they please, for both sides are against them: if Moses was not the author, the books are without authority; and if he was the author, the author is without credit, because to boast of meekness is the reverse of meekness, and is a lie in sentiment
Here is another favorite self refutation. This is from the creationist "scientist" Henry Morris:
Thus there was no death before sin entered the world. The finished creation was "very good" (Genesis 1:31), with an abundance of food and all other provisions for man and animals. There was certainly no struggle for existence, or survival of the fittest, for every creature was created fit for its own environment. When Adam sinned, God brought the curse of decay and death not only upon Adam, but also upon his dominion (Genesis 3:17-20, 1 Corinthians 15:21-22, Romans 8:20-22).
In this passage, Morris tells us that there was no decay or death in the world - but that people still ate food which is in this case would presumably be plant life, a living substance that died and decayed (the process wherein organic substances are broken down into simpler forms of matter) when it was consumed. One cannot deny that there was decay and death if one maintains that there was 'food', unless of course, one redefines what the terms 'decay' and 'death' mean. (See the ad hoc fallacy)
Stolen Concept Fallacy[]
The fallacy consists of the act of using a concept while ignoring, contradicting or denying the validity of the fundamental characteristics or axioms on which it logically and/or genetically depends.
Example written by Nathaniel Branden:
One of the most grotesque instances of the 'stolen concept fallacy' may be observed in the prevalent claim-made by neo-mystics and old-fashioned mystics alike-that the acceptance of reason rests ultimately on "an act of faith." Reason is the faculty that identifies and integrates the material provided by the senses. Faith is the acceptance of ideas or allegations without sensory evidence or rational demonstration. "Faith in reason" is a contradiction in terms. "Faith" is a concept that possesses meaning only in contradistinction to reason. The concept of "faith" cannot antecede reason, it cannot provide the grounds for the acceptance of reason-it is the revolt against reason. One will search in vain for a single instance of an attack on reason, on the senses, on the ontological status of the laws of logic, on the cognitive efficacy of man's mind, that does not rest on the fallacy of the stolen concept. This fallacy must be recognized and repudiated by all thinkers, if truth and reality are their goal. In the absence of such recognition and repudiation, the gates are left open to the most lethal form of mysticism-the mysticism that postures as "science." Who are the neo-mystics' victims? Any college student who enrolls in philosophy courses, eagerly seeking a rational, comprehensive view of man and existence-and who is led to surrender the conviction that his mind can have any efficacy whatever; or who, at best, gives up philosophy in disgust and contempt, concludes that it is a con game for pretentious intellectual role-players, and thus accepts the tragically mistaken belief that philosophy is of no practical importance to man's life on earth.