# 21 Common logic errors

1. Ad Hominem Argument An argument that counters another’s claim or conclusion by attacking the person, rather than attacking the argument itself. (Latin, “against the man.”) A specific example of the Genetic Fallacy which assumes that an idea is not true because of its origin. E.g., a Democrat (or Republican) has a idea, therefore, it must be bad. (Also called the Fallacy of Irrelevance.)

2. Argument from Antiquity An argument is true because it has been held for a long time. Related to the Argument from Numbers, the argument that because many people think something is true, it is true (also known as the Argument Ad Populum).

3. Argument from Authority Stating that a claim is true because an authority (person or group of people) says it is true.

4. Argument from Final Consequences A claim is true because of a purpose or outcome that is served (or vice versa). Also known as a Teleological Argument. E.g., “evolution cannot be true because accepting it will lead to immorality.”

5. Argument from Ignorance The claim that a specific belief is true because we don’t know that it isn’t true.

6. Argument from Personal Incredulity Because you, personally, cannot understand or accept a proposition it is, therefore, not true. Often coupled with a False Dichotomy (see below) as in “I don’t see how the eye could have evolved, therefore, God did it.”

7. Begging the Question or a Tautology A statement that hinges on A=A (or A=B therefore B=A), which is simply restating the premise. For example, “he is unintelligent because he is stupid” or “only a criminal would commit a crime; the fact that criminals commit crimes is proof of this.” (Tautology, literally a repetition.) The “proof” is a restatement of the premise.

8. Confirmation Bias Noticing only the facts that support your thesis but ignoring those that do not.

9. Confusing Correlation with Causation Assuming cause and effect for two variables because they are correlated. E.g., “heroin addicts drank milk as a child, therefore, milk causes heroin addiction.” This is similar to the Post-hoc Ergo Propter Hoc fallacy (see below).

10. Confounding the Unexplained with the Unexplainable Because we do not currently have an explanation for a phenomenon does not mean that it is forever unexplainable or that it requires a paranormal/supernatural explanation.

11. False Continuum The idea that because there is no definitive demarcation between two extremes, the distinction between the extremes is not real or meaningful. E.g, because there is a fuzzy line between science and pseudoscience they are the same thing. Related to the Slippery Slope (see below).

12. False Dichotomy Arbitrarily reducing a set of possibilities to only two.

13. False Premise An incorrect/untrue underlying assumption. Often found in an argument that is otherwise logically consistent but which leads to a false conclusion.

14. Inconsistency Applying specific criteria or rules to one belief, claim, argument, or position but not to others.

15. Moving the Goalpost A method of denial by arbitrarily moving the criteria for proof, acceptance, or rejection out of the range of whatever evidence currently exists or is agreed upon.

16. Non-Sequitur This refers to an argument in which the conclusion does not follow from the premise. In other words, a logical connection is implied where none exists. (Latin, “doesn’t follow.”). Really, all logical fallacies are non-sequiturs.

17. Post-hoc Ergo Propter Hoc This fallacy follows the basic form of: A preceded B, therefore A caused B. This argument assumes cause and effect for two events just because they are temporally related. (Latin, “after this, therefore because of this.”)

18. Slippery Slope The argument that a position is not acceptable because accepting the position means that the extreme of the position must also be accepted. But, moderate positions do not necessarily lead down a slippery slope to the extreme. Careful here: Reductio Ad Absurdum (Latin: "reduction to the absurd"), a form of argument in which a proposition is disproved by logically following its implications to an absurd conclusion, can be a valid argument.

19. Straw Man Arguing against a misrepresentation or over-simplification of the position actually held by an opponent.

20. Special Pleading (Ad Hoc Reasoning) The arbitrary introduction of new elements into an argument in order to amend them so that they appear valid. E.g., “ESP doesn’t work in the presence of skeptics.” A subtle fallacy which is often difficult to recognize.

21. Tu Quoque To reject a position because someone (inconsistently) holds it. Also called an Appeal to Hypocrisy. Person 1: “Smoking tobacco is bad for you.” Person 2: “You smoke, therefore, your argument is invalid.” (Latin, “you too.”)

All this was taken from http://www.unifreethought.com/2011/03/avoid-logic-failz.html

Views: 231

Tags: 21, errors, logic

Comment by Heather Spoonheim on March 30, 2011 at 12:43pm
Thanks, Loop Johny.  I'll keep this on hand - well, bookmarked.
Comment by Loop Johnny on March 30, 2011 at 1:46pm
If someone can take his time to give an example and/or explain each fallacy, I will be very grateful.
Comment by kris feenstra on March 30, 2011 at 4:08pm

Well, I've been taking a stab at it, but I find it hard to create good, clear examples of each.  It's easier to spot fallacies in other people's arguments or to commit them accidentally than it is to intentionally create them yourself.

one through seven:

1. Ad Hominem Argument - The important thing here is that an ad hominem fallacy seeks to discredit a statement or argument by discrediting the person making the argument. In its most stripped down form: Jim says that two plus two equals four, but Jim is horrible at math and got an 'F' on every addition test; therefore two plus two does not equal four. The fallacy is never that direct in practice.

Jim testifies in a court of law that he witnessed Bill commit a murder. Bill's lawyer brings up the fact that Jim is a crack cocaine addict in an effort to discredit Jim's testimony. While it's true that crack addicts are not considered reliable, there is no direct line of logic between Jim being a crack addict and his testimony being false. As the argument stands, Jim's drug habit is irrelevant information and an ad hominem fallacy.

2. Argument from Antiquity - Pretty straightforward. People have held 'X' to be true for so long that it probably is true. Many people talk about how old the Bible is as if it lends credibility to the document. If Bible held true without being proven false for over a millennium, it must be true. By that logic, the older a surviving belief system is, the more likely it is to be true. Someone can correct me if I'm wrong, but wouldn't that make some subgrouping of Hinduism the truest belief?

3. Argument from Authority - It's a lot like an ad hominem fallacy in reverse.  An argument should be considered valid because the person making it is considered credible.  The worst abuse of this in modern times is with academic degrees, especially doctorate degrees.

Dr. Johnson disagrees with the theory of evolution. Because he has a doctorate, it is implied that we should trust him. In most cases where this tactic is used, looking up 'Dr. Johnson' would reveal that his doctorate is completely unrelated to biology, let alone evolutionary biology. His doctorate is irrelevant. Even if he did have a doctorate in evolutionary biology, that alone still would not make his argument correct. The only thing that will make his argument correct is evidence coupled with sound logic.

4. Argument from Final Consequences - X causes Y or has implications on Y. If I accept Y, I evaluate the validity of X based on my acceptance of Y. If I deny or dislike Y, I evaluate the validity of X based on my denial of Y.

For example, I can make a case for causality. That case should be weighed on the merit of its supporting arguments and evidence. Incidentally, causality has (or appears to have) negative implications on the existence free will. If a person rejects causality based only on their acceptance of free will, they have committed a fallacy. The fact that they believe in free will does not necessarily negate the evidence and arguments in support of causality. Free will could be wrong. Causality could be wrong. The understanding of the relationship between free will and causality could be wrong. The perception of incompatibility could be wrong. Both the current models of causality and free will could be partially wrong. Regardless, the situation needs to be evaluated based on the supporting evidence and argumentation, and not on a preference for one idea over the other.

5. Argument from Ignorance - God of the gaps is probably the most common example atheists come up against. I also consider it an argument from ignorance if a person asserts that all propositions are equally likely on the basis that no conclusion has been reached. For example, I cannot perfectly explain or prove abiogenesis. A biblical literalist may say that, in the absence of conclusive evidence for abiogenesis, it is just as likely that God did it. That's a fallacy. Even if I don't have conclusive evidence, that doesn't mean the current evidence does not lean more strongly towards one explanation over the other. One proposition is more likely to be true than the other, so they cannot be held in equal regard as if it was a coin toss.

6. Argument from Personal Incredulity - Sometime this manifests as a simple unwillingness to accept a proposition because the argument is not understood. "Evolution is wrong because it isn't even scientific," often gets quickly whittled down to 'I disbelieve evolution because I don't understand the scientific method or, more specifically, the science of evolution.' The speaker usually says 'X does not make sense,' when they really mean to say 'X does not make sense to me.' Leaving off the 'to me' make sit seem like the fault is with X and not the speaker.

7. Begging the Question or a Tautology - The conclusion is supported by the premise while the premise is also supported by the conclusion. The tricky thing is, there can be any number of steps between the premise and the conclusion. In some cases, the premise and the conclusion are actually the exact same thing (tautology).

example 1:
The word of God is infallible.
We know this to be true because it is written in the Bible.
We know the Bible to be true because it is the infallible word of God.

example 2:
Abortion is murder. Abortion is a crime because murder is illegal.

There's more than one fallacy there, but the second sentence is purely tautological. Murder is defined as unlawful killing. Anything that is murder is inherently illegal and a crime.

Comment by Bryan B on March 30, 2011 at 8:44pm
Misrespesnting and abusing language is another very common one. Like confusing faith and knowledge or calling atheism a religion
Comment by Heather Spoonheim on March 30, 2011 at 8:57pm
On that note, Bryan, I have seen people making multiple assertions about the meaning of words/phrases, constantly asking "do you agree", as though there is some intellectual merit in the ad hoc construction of a Rube Goldberg grammatical mechanism.  Is there an actual 'name' for that sort of bs, or can I just coin it as "Argument from Obfuscation" a.k.a - the Chewbacca Defense.
Comment by Bryan B on March 30, 2011 at 9:47pm
I call it the Redefinition Game, you redefine words to suit whatever it is ya want.
Comment by kris feenstra on March 30, 2011 at 11:19pm

There are a number of verbal fallacies.

• Equivocation - using more than one meaning for a single instance of a word.
• Amphiboly - playing off the ambiguity of grammatical structures that are open to more than one interpretation
• Accent - changing the emphasis of a word or syllable to change the implication
• Composition - falsely assigning attributes of the parts that make up a whole to the whole itself
• Division - falsely assigning attributes of the whole to the parts/ units from which it is made
• Figure of Speech - broad category, but it generally involves taking terms out of proper context e.g. treating technical usage as general usage; treating figurative or rhetorical usage as literal usage; treating abstract usage as concrete usage

Another important fallacy where language is concerned is the etymological fallacy.  This fallacy is committed when contemporary usage of a word is erroneously restricted by historical or original usage.

Comment by Lee Davis on March 31, 2011 at 11:21am
A catholic friend once tried to convince me of the accuracy of the gospels with an ad hominem argument. He stated, in essence, that the apostles were all well-regarded, respectable men in their day and that it would have been unlikely that they would have staked their reutations on the the positions they were asserting without good reason. Leaving aside all of the many arguments one could make about the existence of those men and so on, the basic premise is a classic ad hominen argument. These guys were top drawer follows, and they wouldn't lie.
Comment by Lee Davis on March 31, 2011 at 11:23am
By the way, LJ, thanks for posting this. It's excellent.
Comment by Bryan B on March 31, 2011 at 2:51pm
Thanks for the verbal fallacies Kris I have never seen those before. I had always just categorized language problems all under one category. So, thank you posting that.

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