Saturday, February 15, 2014

Fallacious Arguments

from various sources

Fallacies are statements that are logically false, but which often appear to be true. Aristotle, in his Sophistical Refutations (Sophistici Elenchi), identified thirteen fallacies, as follows:

Linguistic fallacies
  •     Accent
  •     Amphiboly
  •     Equivocation
  •     Composition and Division (two sides of the same coin)
  •     Figure of Speech
Non-linguistic fallacies
  •     Accident
  •     Affirming the Consequent
  •     In a Certain Respect and Simply
  •     Ignorance of Refutation
  •     Begging the Question
  •     False Cause
  •     Many Questions

Here are most of the known fallacies, in alphabetical order, with their relative fallacies listed beneath them, including a deeper look at what these fallacies mean (stars indicate fallacies identified by Aristotle):

Accent*: Emphasis that changes the meaning of the sentence.
  • Emphasis
Description
The accent (or emphasis) used within the statement in question gives a different meaning from that of the words alone.
The key principle is that emphasis put on a word or phrase directs attention to those words, signifying importance.
Emphasis in speech may be accidental or due to dialect, but often reflects the deeper meaning of what the person really intends.
Example
I wonder if you really want to do this. (Accent implies 'you want to do this')
What do you think people need about Charmix? (Accent says 'you need Charmix')

Discussion
The emphasis put on words in a sentence changes the meaning, often radically, which is one reason why the spoken word can communicate so much more than the written word (although limited emphasis may be used here).
Emphasis draws attention to words, indicating priority, although this often happens at a subconscious level (which is one reason it is often used in subtle sales and advertising pitches).
Because of the subconscious element, it is possible to understand what a person really means, and what they are actually feeling, from the emphasis they use.
We also interpret the emphasis subconsciously, which is an opportunity for the persuader to turn simple words into a powerful way of subtle communication.
But this is not always the case. Sometimes emphasis is placed on a specific word or set of words for emphasis sake only; not to give a subtle ulterior message as seen in the second example above.
Accident*: A general rule used to explain a specific case not covered by it.
  • Destroying the Exception
  • Dicto Simpliciter
  • Sweeping Generalization
Description
X is explained by rule Y. But X does not fall under Y.
A general rule is used to explain a specific case that does not fall under its rule.
Example
Rich people like to receive good service. You therefore must like good service.
You can't go there, Mr. President. Nobody is allowed in.
It is wrong to hurt people. You should not have hit that person who was attacking you.

Discussion
What often happens here is that the general rule being used is either assumed to have a wider scope (being more general) than is reasonable, or that it is simply mismatched with the case that it is being used to cover.
Accident often appears to be using deductive reasoning and hence seems to carry reasonable logic.
We have a deep need to explain things that happen, which leads to many people accepting a general rule as explanation for a specific case, even when that rule clearly does not apply. A convenient Accident may thus be used deliberately when there is no general rule available.
Affirming the Consequent*: If A then B. B is true, so A is true.
  • Non Sequitur
Description
If A is true then B is true. B is true. Therefore A is true.
If B follows A, then you can assume you can go back the other way also.
Example
I am in London, England. I am in England, therefore I am in London.
If you are cheating on me, you will be out of the house a lot. You are out of the house a lot, so you must be cheating on me.

Discussion
This assumes that an if...then... statement is commutative, that given 'If A then B', you can also reverse it to 'If B then A'. The B, or 'then' part of the statement is called the 'consequent' (the A is the antecedent).
Amphiboly*: A sentence has two different meanings.
  • Amphibology
Description
The structure of a sentence has more than one possible meaning.
Example
Young men and women. (are the women young?)
Who is the doctor. (it's Doctor Who!)

Discussion
As with other forms of ambiguity, amphiboly can cause confusion and hence puts the other person into a state where they are open to different ideas.
Amphiboly may be deliberate or accidental. Where it is deliberate, it may be used to confuse or make subconscious suggestions. This is particularly effective where the second meaning of the sentence may take a few moments to sink in. Thus the obvious meaning is stated with the intent that the secondary meaning is interpreted only at the subconscious level.
A common form of amphiboly is where an adjective is used with two nouns (e.g. 'Good boys and girls'), making it unclear whether the adjective applies to the second noun.
Appeal to Authority: Referencing an 'expert'.
  • Ad Verecundiam
Description
An expert asserts A is true. Therefore A is true.
The expert, of course, may not be expert, but they are a touchstone that people use to avoid having their own expertise challenged.
You can also assert your own expertise. If the other person cannot challenge your credentials, then they cannot challenge your argument.
Example
Mike said that this train will be late.
Well, you know what they say...
In a survey, 80% of doctors agreed that this drug can be very effective.
I've been doing this for twenty years, you know.

Discussion
The problem with this appeal is not so much in the assertion of truth but in the true expertise of the so-called expert, who may be guessing or even joking. It is also known that if you bring together a group of experts then you are likely to get less than full agreement about any given question.
The expert may not be named (and is hence an anonymous authority) or may be absent and unable to answer probing questions. In this case, it is not known whether the person quoting the expert is quoting them accurately or even making the whole thing up.
Appeal to authority is a common method used in confidence tricks, where the confidence trickster sets themself up as an authority and so both dissuades the target from asking questions and encourages them to trust their 'expert' judgment. 
Appeal to Common Belief: If others believe it to be true, it must be true.
  • Ad Populum
  • Appeal to Belief
  • Appeal to Majority
  • Appeal to Popularity
  • Bandwagon
    • Ad Populum
  • Value of Community
Description
If something is believed to be true by a lot of people then it must be true.
A variant is where the probability of truth is assessed by the number of people making the assertion (especially when you can see who is voting for and who is voting against the idea).
Example
Oh come on, everyone is saying that this is the right thing to do.
Your family all like the car...
In a survey, 8 out of 10 doctors agreed that this drug is dangerous.

Discussion
When we are uncertain about something, we turn to other people and assume they know what they are doing. We do the same with beliefs. The more other people believe something, the more likely we will be to accept that it is true (especially if we tend towards black and white thinking).
Appeal to Common Practice: If others do it, it must be ok to do it too.
  • Ad Numeram
Description
Many people do X. Therefore X is right.
If other people do something, then it is a reasonable thing to do.
Example
They were doing it, so I thought it would be ok to do it as well.
Oh come on, it's common practice!
Well then, are you going to join in, or what?

Discussion
This is often used as an excuse. When we are uncertain about something, we turn to other people and assume they know what they are doing.
This is also a method of influence in social groups, where pressure is put on people to conform.
Adoption of common practice happens when people are uncertain what to do. When the right thing is unclear, it is generally safer to copy others. 
Appeal to Emotion: If it feels good, it must be true.
Description
X makes me feel good. Therefore X is true.
When something is associated with good feelings, then it is desirable and must be true. The converse is also true: when something is associated with negative feelings, then it must be wrong and bad.
Example
Our new cutlery set will make you feel so good when you see it laid out on your dining table. 'Style and Grace' is the right thing for you.
The Maki people of the South are known to be invading our towns! They are corrupting our children and taking our jobs!! Vote for me and I will eradicate this menace!

Discussion
The arousal of emotion is known to smother rationality, hence if it is introduced into an argument, then it is more likely that logical reasoning will be ignored. Many arguments thus deliberately seek to evoke emotions of the listeners.
The word 'good' is built into the language to support this. We talk about 'good feelings' and 'bad feelings' when good and bad are really about values. This association makes it easier still to bring emotion and value-based decisions together.
Appeal to Equality: Assertion deemed true or false based on assumed pretense of equality.
Appeal to Fear: Gaining compliance through threat.
  • Ad Baculum
  • Ad Metum
  • Appeal to Force
  • Consequences
  • In Terrorem
  • Scare Tactics
Description
X is presented. It causes fear. Therefore Y (which has some relationship to X) is true.
Mostly, this is done by some form of threat, which may be to the person or may be to something about which they care. The threat may be physical, emotional or spiritual.
Example
I know where you live, and I have friends who like a good fire...
Those who do not rejoice will burn in hell.
If Mike heard you saying that, he would not be very happy.

Discussion
Fear is a basic motivator that is used surprisingly often. The danger of using this is that it can also lead to other irrational emotions that can rebound upon the persuader, such as dislike and hate. Fear also results in people running away, but not always in the direction intended.
Fear appeals seldom gain more than compliance. Emotional and intellectual agreement will lag far behind and may never be gained (in fact opposing ideas may be strengthened).
Appeal to Flattery: Make them feel good.
Description
You are a nice, good person. Nice people accept my arguments. Therefore you will accept my arguments.
Compliment the other person. Make them feel good about themselves. Show that you approve of them and their actions. Tell them that they look good.
Then ask them to agree with you.
Example
Hey, you're looking great today. You know I read your paper and it was amazing. Now doesn't it make sense for us to go out tonight?
You're the kind of person who understands this. Could you endorse me?
That's a really good idea. So let's go and show it to Bill. We can also ask him about next year at the same time.

Discussion
Flattery is always nice to receive as it strokes a person's sense of identity. In doing so, it leads them to like you in exchange and want to repay you for your kindness. Agreeing to your request is an immediate way of doing this.
Flattery can include compliments on such as:
  • Their looks
  • Their actions
  • Their beliefs or values
  • Their intelligence, ideas and thinking
  • Their creativity and aesthetic sense
  • Their ability to argue
  • The things they have produced
Some people are more susceptible to flattery than others, particularly those with a weaker self-image and who may be generally less confident.
Flattery is a particular kind of Appeal to Emotion.
Appeal to Novelty: Newer is better.
  • Ad Novitam
Description
X is new. Therefore it is better than that which it seeks to replace.
Newer is better. X is always better in all ways than X-1.
Example
This new YPod is much better than the old XPod.
This year's model has twenty new features.

Discussion
This is an assumption of the modern (or maybe not so modern) notion of progress, that just because something is new it is good and superior and can replace what has gone before.
Novelty is one of the forces of fashion (although social approval is the real founding force).
We live in an age of hope, where we believe that technology and progress will save us from the problems we are creating. Global warming, overcrowding, disease and more afflict us, yet we continue like lemmings, in the blind belief that science will find the way, as it has done so for the past century or so. Anything new is hence greeted with interest and sometimes a hint of desperation...
Against novelty, some people become weary of the new, having long experience of new things being badly designed and full of defects. It also disturbs the comfort of the tried and true and requires cognitive effort to learn new ways.
Appeal to Pity: Going for the sympathy vote.
  • Ad Misericordiam
  • Appeal to Sympathy
Description
Please feel so sorry for me or my cause that you agree with me.
Gain agreement by sympathy or empathy. Show how you have a deserving cause. Get the other person to put themselves in your shoes and see your sorry plight. Use pictures, testimonies, and other evidence, all turned to your purpose.
Example
I have hurt my leg. Can you drive, please?
Just look at these pictures of the children. Can you donate something to help them?
I haven't completed my work as my dog just died.

Discussion
Appeal to Pity often uses values as emotional levers to gain compliance. This can be particularly powerful, as it is a strong social force. It is about 'being good' and can easily lead to further commitment. Appeal to Pity or Appeal to Sympathy is used frequently by homosexuals to gain social compliance, and if that does not work they vilify the other person by calling them a "homophobe" (the fallacy of Judgmental Language; using insulting or pejorative language to influence the recipient's judgment), another manipulative tactic to force social compliance.
Appeal to Ridicule: Mocking the other person's claim.
  • Ad Absurdum
  • Ad Ridiculum
  • Reductio ad Absurdum
  • Reductio ad Ridiculum
Description
X is amusing, absurd. Therefore it is false.
Mock the other person's claim and argument. Make fun of it. Get people to laugh at it.
Alternatively, mock the alternatives that they might choose, giving them only one option that you have not mocked.
Example
Supporting that cause would take several surgical trusses!
Those other cars look ridiculous. This is the only man's car here.
Those clothes would make you look like a overdressed donkey.

Discussion
Ridiculing something is to place it at a lower social position. If a person is associated with that thing, then they, too, are moved to that lower position. When others see a person in a lower social position, they will not associate themselves with that person, for fear of being dragged down to that position. The original person knows this, and will seek to avoid loss of social status.
The ridiculed thing is thus poisoned and made undesirable, and people will distance themselves from it.
Appeal to Tradition: It has always been done this way, so this way is right.
  • Ad Antiquitatem
Description
X has always been done. Therefore X is right.
Claim something to be well-established and proven. Say that it is traditional, and that to change it would be sacrilegious or very wrong in some way.
Example
My father and his father before him polished wood this way. Don't tell me how to polish wood.
We've been doing this for thirty years, and we've never had problems with it.
The tradition in this town is to buy from local traders.

Discussion
Tradition, once established, becomes a cultural thing, where people do it without thinking and defend it simply because it now is a part of the woodwork. Familiarity breeds both ignorance of the true value of something and a reluctance to give up the 'tried and true'.
Argument from Ignorance: Accepting circumstantial evidence.
  • Ad Ignorantium
  • Appeal to Ignorance
  • Burden of proof
  • From Ignorance
Description
Nothing is known about A. Yet a conclusion is drawn about A.
Facts may be given all around a particular subject, yet nothing specific is said about the subject. Based on this circumstantial evidence, it is assumed that something may be known about A.
A variant occurs where a lack of evidence is assumed to be proof, for example when a murder suspect does not have an alibi.
Example
You live on Sunny Street. You have a gun. Nobody else on Sunny Street has a gun. There was a murder on Sunny Street last night. You were involved.
You live on Sunny Street. You have a gun. The person was knifed. You were not involved.

Discussion
Circumstantial evidence is well known in the courtroom as being very weak evidence, if evidence at all. Yet in daily life it is used with impunity. Yet the notion of a person being innocent until proven guilty also makes conclusions without proof. Similarly, scientists largely assume something does not exist until it is proven.
A significant question with this is where the burden of proof lies. Is it with the prosecutor or the defendant? Usually it is with the person making a claim that something exists or has happened.
Assertion: What I say is true.
  • Alleged Certainty
Description
I say that X is true. Therefore X is true.
The mere fact of asserting a truth makes it true.
Example
You are stupid.
This shirt is the best that money can buy.
Vote for Jimmy. He will save this country!

Discussion
Being assertive is an adult way of behaving, stepping off the passive-aggressive continuum and stating a truth with conviction. This can be a very persuasive position, as it removes aggressive threat without capitulation, demonstrating maturity and apparent wisdom.
Assertion is often a veiled appeal to authority in that it makes the assumption that the person making the assertion is an expert or has a position of unassailable formal authority.
Association Fallacy: Guilt by association; two things sharing a property are the same.
Attack the Person: Distracting them from their argument.
  • Abusive Ad Hominem
  • Ad Hominem
  • Ad Hominem Abusive
  • Circumstantial Ad Hominem
  • Genetic Fallacy
Description
Attack the person in some way. For example:
  • Attack their expertise, questioning their qualifications or experience
  • Criticize their physical appearance or dress
  • Comment on their inability to make a good argument
  • Point out their junior status
  • Attack their values as being contrary to social norms
  • Interpret a minor error as major
  • Attach them to discredited others
Example
You are not qualified to make such a statement.
You would say that, wouldn't you.
And who do you think is going to believe you.
Of course you will defend your own department.
You mean you have not considered Wikkin's work? That is a serious omission.
Ew! Did you know flip-flops were invented by hippies?

Discussion
Attacking the Person is a form of distraction, forcing them into defending themself and away from their argument. Most people, when personally attacked, respond with a fight-or-flight reaction and so either jump to their own defense or cognitively flee (and in doing so, drop any argument they are making).
Attacking people in public frames you as an aggressive person who attacks those who oppose you. Other people there will consequently be less likely to attack you or use strong arguments against you, for fear of being attacked by you, hence strengthening your power position.
A sub-division: Abusive Ad Hominem is where the person is attacked and discredited. Circumstantial Ad Hominem occurs where an excuse is made for the person which negates their argument due to some special circumstances, such as the role they have.
The genetic fallacy is condemning an argument because of where it began, how it began, or who began it.
Begging the Question*: Circular reasoning to prove assumed premise.
  • Chicken and Egg argument
  • Circular Definition
  • Circular Reasoning
  • Circulus in Demonstrando
  • Petitio Principii
  • Reasoning in a Circle
Description
The truth of A is assumed within the original premise about A. Hence A is not really proven by the argument.
This may occur through a simple statement or via a more complex set of statements that go around in a circle and eventually 'prove' the original statement to be true.
Another variant is: If A is not wrong, then it is right.
Example
I am not a liar.
This restaurant serves the best food in the town, because it has the best chef. It has attracted the best chef because it has the best reputation. It has the best reputation because the chef cooks the best food.
You are not bad, therefore you must be good.

Discussion
Begging the Question does not really answer it outside of its own assumptions. This happens when people accidentally or deliberately start from an unproven position and try to use this to prove the position. Like a house built on sand, the argument does not stand up to a light push.
Cherry Picking: Pointing at individual cases/data that seem to confirm a position, while ignoring significant portions of cases/data that contradict that position.
  • Incomplete Evidence
  • Stacking the Deck
  • Suppressed Evidence
Description
This occurs when an individual ignores or dismisses significant portions of cases, data, or context that stand in direct contradiction to the position they are attempting to defend.
Example
There is scientific evidence that homosexuals are born that way because:
  • with identical twins, both are homosexual
  • with identical triplets, all are homosexual
  • with identical triplets where one is female and the other two are male (or vice versa), either all are homosexual or just the two of the same gender.
Discussion
If you write a significant amount of information and someone selects one or two single sentences from it to respond to, while glossing over or ignoring the rest of it, either refusing to try and answer it or unable to answer it, this is an example of Cherry Picking. With the above example, a form of Stacking the Deck, the cases of scientific evidence to the contrary are deliberately overlooked or dismissed entirely, where:
  • with identical twins, only one is homosexual while the other is not
  • with identical triplets, only one is homosexual while the other two are not
  • with identical triplets where one is female and the other two are male (or vice versa), only one of the three is homosexual while the others are not, or one of the two with the same gender is homosexual while the other is not.
Complex Question: two questions, one answer allowed.
  • Double Bind
  • False Question
  • Loaded Question
Description
X and Y are unrelated questions. They are combined into question Z, which requires a single answer.
Example
Have you stopped smoking? ['yes' or 'no' both admits being a smoker]
Will you help me and carry this?

Discussion
The Complex Question often is arranged such that whichever way you answer the question, the questioner gains the advantage (i.e. a double bind).
Composition*: Generalizing from a few to the whole set.
  • Faulty Induction
  • Generalization
Description
If A is X and B is X then the group to which A and B belong are all X.
Given a number of unrelated items, they are taken as being members of a distinct group, about which general attributes may be identified.
These attributes may well be taken from a limited number of items or parts and generalized up to the whole.
Example
All people in this town are idiots.
These two tools are blunt. The whole box of tools must be blunt.
Oxygen and hydrogen are gases at room temperature. Therefore water (H20) is a gas at room temperature.

Discussion
This seeks to use inductive reasoning, but does so falsely, generalizing when there is no sound rationale for doing so. This is the basis of stereotyping, which is a Composition fallacy.
The generalization that is taking place may be due to sloppy thinking or may be a deliberate way of seeking a general rule (which may then be applied deductively elsewhere).
Conspiracy Theory: Re-frame refutation as further proof.
  • Cancelling Hypotheses
Description
A is true. B is why the truth cannot be proven. So A is true.
Make a statement. Then explain why it cannot be proven. Accuse anyone who challenges the second statement of trying to cover up the truth. Use this attempt as proof that the original statement is true.
Example
Flying saucers have landed. The government is covering it up, which is why there is no evidence about it. Of course they deny it!
The senior managers in this company are bleeding it dry. But then they control the accounts, which is why finances seems to be healthy.

Discussion
This fallacy works by making it impossible to challenge the proving statement without proving it. The focus of attention is thus moved to the person trying to disprove the 'proof', and re-frames their refutation as further proof.
Denying the Antecedent: If A then B. A is false, so B is false.
  • Non Sequitur
Description
If A is true then B is true. A is not true, therefore B is not true.
To disprove something, show how it can be caused by something else. Then show that the cause does not happen (then assume that this proves the antecedent is also false).
Example
If you give a man a gun, he may kill someone. If he has no gun, then he will not kill anyone.
If you work hard, you will get a good job. If you do not work hard you will not get a good job.
I am in London, England. I am not in London, therefore I am not in England.

Discussion
In an 'If A then B' statement, A is the antecedent and B is the consequent.
When you know that 'If A is true then B is true', this statement is only valid for truth of A and B. If A is false, then it does not necessarily follow that B is also false. A place where this is true is in Boolean logic, where A and B are binary variables and can only be true or false. In life, there are often situations where A and B can have many other states.
Division*: Assuming the parts have the characteristics of the whole.
  • False Division
  • Faulty Deduction
Description
X has characteristic Y. P is a part of X, so also has characteristic Y.
The general assumption is that the parts of a system all have characteristics of the whole system.
Example
Welsh people speak Welsh. You are Welsh -- so you can speak Welsh.
Cars go fast. A seat is a part of a car. So seats go fast.
Tomatoes are red. So the pips inside are also red.
You are work in Hewlett Packard, which makes computers. So you can make computers.

Discussion
There are two ways for this to fail: First, the assumed characteristic of the group may not be true for all members and, secondly, the characteristic that is true of the whole is not applicable to the individual parts. This latter case happens when the individual parts create a whole, where the whole is more than the sum of the parts.
This fallacy fails when the unspoken assumption about the characteristics of the group are not true for all members of the  group. This is typically true of groups of people, where the attribution of generalized characteristics form a stereotype, which is then applied to individual members.
Going from the general to the specific is deductive reasoning. Division seeks to use this rational process, but does it in an inadequate and invalid way.
Equivocation*: A single word with more than one meaning.
Description
This happens where the same word is used, but has two or more different meanings, leading to easy confusion as at least one of the meanings is likely to be false.
Example
The weather forecast is for high winds. We're ok as we are in a valley.
The honey is set on the table.
He is a cut above the rest.

Discussion
Equivocation can be accidental, and it can be deliberate. When we communicate, one person attempts to send a message and the other attempts to interpret the original meaning. When the perceived meaning of individual words is different from that which is intended, either the whole sentence is given new meaning or it loses all meaning. The latter is generally undesirable, so we will struggle to create some form of meaning.
Equivocation and other forms of ambiguity lead easily to confusion, which is a mental state where people become open to suggestion. It can thus be deliberately used as a persuasive device.
Excluded Middle: Only extreme views are valid.
  • Black and White Thinking
  • False Dichotomy
  • Polarization
Description
Only extreme views are valid.
Moderation is weak and uncertain. To be valid, an argument must have a clear opposite.
Ignore any central position. Polarize any issues and then select one end of the spectrum. Criticise any middle position as floppy and compromising, which by definition is only half of what could be had.
Example
Women who use Citro perfume will attract men. [Those who do not, will not.]
If we do not save the whales, the world is doomed.
I have an absolute right to carry a gun. Those who oppose that right deserve what they get.

Discussion
Where people have a high need for certainty  and control, extreme views provide what may seem to be a defensible position, as you only need to look in one direction for the 'enemy'. Dividing the world into good and bad, right and wrong also plays to certainty needs, as your friends and enemies can now be clearly identified.
False Analogy: X has property Y. Z is like X. So Z has property Y.
  • False Metaphor
  • Weak Analogy
Description
X has property Y. Z  is like X. Z therefore has property Y.
Use analogical comparisons to connect the item in question to another item that has desired characteristics. You can then claim that the first item has the desired property.
Example
People are like dogs. They respond best to clear discipline.
This soap is like a dream. It lifts you up to a spiritual plane.
A school is not so different from a business. It needs a clear competitive strategy that will lead to profitable growth.

Discussion
Analogy is saying 'A is like B' and is a powerful way of explaining one thing in terms of another. Where it falls down is when A is assumed to be like B in all respects and any attribute or characteristic of B can be unequivocally attributed to A.
In the false metaphor variant, the comparison is metaphoric. As analogies say 'A is like B', metaphors say 'A is B'.
  • Analogy: She is like a dog
  • Metaphor: She is a dog
The effect is still the same: the attributes of the analogy or metaphor are brought back to the original subject. The major difference is that in a metaphor, the equation is more explicit and direct.
The typical fallacy in this is that the comparison is not a good one and creates significant falsehood.
False Cause*: A causes B (but no proof).
  • Cum Hoc Ergo Propter Hoc
  • Questionable Cause
Description
A causes B (without real proof that this causal relationship actually exists).
This causal relationship is often claimed when there is correlation between A and B (that they vary together) or a relatively distant causal connection.
Example
It is dark now, which makes it very dangerous. [It is not the dark that causes danger].
Drinking fresh water will keep you well. [It may contribute, but it is not the only or sufficient cause].
Money makes people arrogant. [Not all people, and not always just money]

Discussion
Cause-and-effect reasoning is a valid form of rational logic, but only if the causal relationship is established. It is very easy to find that two things vary together and assume cause-and-effect, but this only proves correlation. It may be, for example, that both are effects of a prior common cause.
Causal arguments are often wishful thinking, where the speaker is seeking to prove their case, and hopes (as with other fallacies) that their causal assertion is not challenged.
False Choice: Choice is A or B. Rejecting A is selecting B.
  • Bifurcation
  • Either/Or
  • False Dichotomy
Description
Either A or B is true. If A is true, B is therefore false. C is not an option.
The other person is offered a choice where rejecting one item acts as a selection of the other.
Example
Either you are with me or against me.
We have to spend less on hospitals, otherwise we won't be able to afford education improvements.

Discussion
This is based on the assumption that the choices offered are the only choices. By focusing on the choice, the decision to be made, the other person is distracted from the fact that there may be other alternatives.
This is usually presented as two choices, although more may sometimes be used.
False Compromise: Extreme views are wrong. The middle way is right.
  • Splitting the Difference
Description
X and Y are opposite alternatives. So Z, a middle path, is the best choice.
Avoid extremes. Seek compromise through a moderate, middle way. Take averages. Assume that any polarized view is automatically wrong.
Example
Environmentalists want us to recycle everything. Capitalists want us to buy everything new. I think we should recycle some things.
The left wing want to help the poor. The right wing want to minimize taxation. We should have means-tested benefits so only the poorest people get help.
Jim wants to go North, Fred wants to go South. So lets go West.

Discussion
It is easy to assume that extreme views, by the very fact that they are extreme and so automatically wrong. The middle way is often a safe choice, and people who are risk-averse will often take this alternative. It is also a view taken by people who see that all views are valid and that a compromise is the best way of getting agreement. This may be true, but it does not make this middle choice correct.
False Effect: A is assumed to cause B. B is proven wrong, so A is wrong.
  • Non Causa Pro Causa
Description
X apparently causes Y. Y is wrong. So X is wrong.
If you want to prove something wrong, find something that it appears to cause, and then prove that the caused thing is wrong. You can also do the reverse to show something to be right.
Example
I pulled on the string and the kite fell to ground. Pulling on the string is therefore ineffective.
Loud music leads to deafness. Turn that music down!
Eating sweets makes you happy. You should eat sweets.

Discussion
This works because attention is distracted from the (incorrect) assumption that X causes Y to the question of whether or not X is right or wrong (which usually cannot be questioned).
When presented with cause and effect, it often seems to make sense simply because of the assertion of causality. This results in people accepting invalid causal arguments. 
Four Terms: All A is B. All C is D. So all A is D.
Description
All A are B. All C are D. So All A are D.
Make two statements and make an unspoken leap that connects these statements to allow a third, conclusive statement to be made.
Example
All dogs are mammals. All fish are animals. So all dogs are animals. [true, but not proven by the first two statements]
Man is an intelligent being. No woman is a man. Therefore no women are intelligent beings. ['man' has two different meanings]

Discussion
Syllogisms should have only three terms, with one term being the bridge between the major and minor premise that forms the conclusion. So where four terms appear in the major and minor premises, these two statements are logically disconnected and no logical conclusion may be drawn.
Sometimes it may appear that there are three terms, as in the second example above. This can still cause problems where one term actually has different meanings (equivocation) in either term. For example, 'man' can mean 'humanity' or 'male'.
Gambler's Fallacy: Chance can be predicted.
Description
Chance is affected by more than random events. It can be controlled by luck, skill and specific identified events. When you hit a 'lucky' patch, you just cannot lose. When the odds are stacked against you, you have no chance.
Example
I've lost three nights in a row. I will win big tonight. I'm wearing my lucky watch, just to make sure.
You know, every time it rains, I've come out without an umbrella and am miles from the car.

Discussion
One of our basic needs is for a sense of control, which we gain by seeking to predict the future and by attributing cause to events that occur. We also seek to win and avoid failure, which further drives us both to explain our losses outside ourselves and also to compensate for losses by trying to end up as a winner.
Habitual gamblers fall headlong into these traps. Many others are affected by it too. For example, 'Murphy's Law' (that when something goes wrong it will be the worst thing at the worst time) is often used to explain and provide comfort when things go wrong. Particularly for the gambler, it also means explaining winnings through external events and lucky charms (most of us prefer to explain winning through personal skills, although where chance is involved, we also fall into the trap). 'Luck' itself is an example, as it does not exist. What is random is random. The laws of statistics are all there are.
When things do go wrong and when our predictions fail, we need to be comforted in some way and the Gambler's Fallacy can be turned to this purpose also. We can explain failures as bad luck rather than incompetence or poor decision-making. We can take comfort in the fact that lady luck has gone out for the evening and there was nothing we could have done to change things.
Hasty Generalization: Generalizing from too-small a sample.
  • Converse Accident
  • Hasty Induction
  • Inductive Generalization
  • Insufficient Sample
  • Insufficient Statistics
  • Leaping to Conclusion
  • Lonely Fact
  • Statistical Generalization
Description
X is true of A, B and C. Therefore X is true of everything.
Find a commonality in a few things and then generalize to assume that it is also true for all things in the same class.
Example
I met some children from Garton yesterday, who were very polite. I think all children from that area must be well-behaved.
I've met three race drivers today and they all were rather aggressive. Clearly, race drivers are all aggressive.
I've tried two Albanian cheeses and they were both rather bitter. Albanian cheese is not really to my taste.

Discussion
We all seek to classify things we experience in order to help make decisions about similar items we meet in the future. This generalization is a form of simplification and always results in some distortion.
If we generalize too soon, we may classify things incorrectly. This is partly due to a lack of real understanding of statistics, where a representative sample needs to be taken before realistic rules can be inferred.
Hasty generalization typically happens when we are in a discussion and are trying to make a point. We do not have the data available to prove something so we jump to conclusions and use what little information we have to 'prove' the point that we want to make.
Illicit Major: All X is Y. No P (which is a subset of Y) is X. Therefore no P is Y.
Description
All X is Y. No P (which is a subset of Y) is X. Therefore no P is Y.
Unspoken assumption: All Y is X.
Example
All Londoners are European. No Parisiens are Londoners. Therefore no Parisiens are European.
Discussion
This is a particular case of a categorical syllogism, where overlaps of sets are taken to be different in each statement. Thus the fallacy in the example occurs when the first statement is assumed to be reversible (that if all Londoners are European, then all Europeans are Londoners).
More formally, the predicate (Y) of the conclusion (no P is Y) refers to all members of that set. Yet the same term (Y) in the major premise (All X is Y) refers only to some of the members (X) of that set.
The 'Major' in the name is the major premise, the first statement in the syllogism.
Illicit Minor: All X are Y. All X are P. Therefore all P are Y.
Description
All X are Y. All X are P. Therefore all P are Y.
Unspoken assumption: All P are X.
Example
All New Yorkers are beautiful. All New Yorkers are intelligent. Therefore all intelligent people are beautiful.
Discussion
This is a particular case of categorical syllogism, where the second statement is assumed to be reversible. In the example, it is assumed that the statement about New Yorkers and intelligence is reversible (all intelligent people live in New York).
More formally, the subject (P) in the conclusion (All P are Y) refers to all members of at category, but the same term (P) in the minor premise (All X are P) refers only to some members of that category.
The 'Minor' in the name is the minor premise, the second statement in the syllogism.
In a Certain Respect and Simply*: Extending assumed boundaries too far.
  • Secundum quid et simpliciter
Description
A is an attribute of B. So A is an attribute of C.
Take an attribute that is bound to a certain area and assume that it can be applied to a wider domain than was originally intended.
Example
A dog has white teeth, so the dog is white.
There is money in my pocket, so there is always money in my pocket.

Discussion
When we discuss an attribute of something or somebody, we implicitly assume that there is some constraining contextual factors. When the assumption is carried too far in this context, then this fallacy is committed.
Insignificance: Making a minor cause seem major.
Description
X is actually one relatively minor cause of Y. Yet X is assumed to be a major cause of Y.
Blow up a minor cause to be a major cause. Make a big deal about it. Show how wonderful or terrible it is.
Example
I was in too much of a hurry to put on my make-up, which is obviously why he did not ask me to dance.
Genetically-modified crops are a terrible threat to human health. [spraying with chemicals is not something I want to talk about]
This stage is far too small. No wonder we are not getting the audiences we deserve.

Discussion
Blowing up an insignificant cause can have two different benefits for the persuader. First, if this minor cause is important to the persuader, then disproportionately high significance is placed on it. The other use is when the persuader wants to avoid talking about another cause. By making a big deal of the minor cause, a distraction away from the unwanted cause is created.
Logical Inconsistency: Arguments that contradict one another.
  • Inconsistency
Description
Multiple statements are given which contradict one another.
These may be given together or may be separated in time. Sometimes the contradictions are rather subtle and are difficult to spot. At other times, they are obvious. If you have enough authority, then you may be able to carry this off.
Example
Sim is cleverer than Jim. Jim is cleverer than Tim. Tim is cleverer than Sim.
Let's all go to the football game tomorrow. My wife doesn't like football. I know you're busy. We'll all enjoy ourselves.

Discussion
Inconsistencies may be found in various circumstances. Often they are used in ignorance, where the person does not realize they are being inconsistent. Sometimes it is just lazy thinking. Sometimes they are used within an emotive context, where the person has lost all sense of logic and is desperately trying to prove something. Another possibility is a deliberate use of inconsistency to confuse the other person and hence distract them from the real game. Finally, sometimes people in authority deliberately contradict themselves because they know they will not be challenged and possibly as a demonstration of power.
Logical inconsistency is different from personal inconsistency. Logical inconsistency is about using arguments which are not internally consistent. Personal inconsistency is about accusing others of inconsistency.
Many Questions*: Overloading them with lots of questions.
  • Plurium Interrogationum
Description
Ask many different questions. They may be related with a central theme. They may also be unrelated.
Example
When and where will you expect me to be and how often do you want this to happen and what will be the time of day and which weeks?
Tell me what you want to do next, and then let me know what we can do tomorrow. I want to know from you exactly what you think about the show we did and also which way it is to the middle of town and whether we should go there today or tomorrow. Oh, and, do you want pizza for tea or will David be home this evening?

Discussion
When listening to a complex question or statement, we have a limited ability to understand everything. This causes confusion and we may stumble through a partial answer or say nothing, letting the speaker pick up again and perhaps answer the question in the way they want, or just to continue.
If unrelated questions are asked, the effect is multiplied as the listener not only tries to remember them but also make sense of them with regard to the relationship between them.
Misleading Vividness: A memorable few events prove high probability.
Description
A few vivid and memorable events occur (even just one). These are taken as proof that the event happens more often than it actually does. The pattern is as follows:
  • X occurs.
  • X is memorable.
  • X is repeatedly thought about.
  • X is assumed to be a common event (or at least more likely than it actually is).
Example
A gruesome night-time murder is covered in the news. People listening become worried and fewer people go out at night, to the extent that restaurants notice a drop in takings.
A student knows several people who got jobs easily after college. They assume they will also get a job without much effort. The student is surprised when they find jobs are not easy for them to get.
A few people are found to be cheating in order to get state benefits. This is played up in a TV exposé and is subsequently taken as proof that there is a 'benefits culture' and that most people on benefits do not deserve such support.

Discussion
The way that this fallacy works is that there is a confusion between recall and occurrence. When an event comes to mind a number of times, each remembering appears as a separate occurrence of the event (even if the person knows well that the event happens infrequently). This biases the person's estimation of probability.
This effect appears in decision errors when the availability heuristic is applied. It may also be seen in sayings such as 'One swallow does not make a summer' (the swallow bird migrates to the UK for the summer season).
After the 9/11 disaster in New York, the pictures of the towers collapsing were shown so often that some people (and particularly children) thought that many towers were collapsing. Fear of further terrorist attack increased massively and much more was spent on security in many areas.
This type of decision is often one where there is a low probability of an event, but where the occurrence of the event would be disastrous or otherwise very unwelcome (sometimes called a 'Black Swan' event). In considering the decision, the anticipated pain of the possible event overwhelms the fact of the low probability.
A reversal of this is also possible, where the desirability of the event makes it seem more likely than in fact it actually is. This is an effect that gamblers face when they believe they are more likely to win than they actually are.
Misleading vividness plays to hope, where the person translates the hope for an event to happen (or not) into a probability of the event.
Missing the Point: Drawing the wrong conclusion.
  • Ignorance of Refutation*
  • Ignoratio Elenchi
  • Irrelevant Conclusion
  • Non Sequitur
Description
A set of statements leads to conclusion X. Yet conclusion Y is drawn.
An argument is given from which a perfectly valid and sound conclusion may be drawn, yet the stated conclusion is something else.
Example
There has been an increase in burglary in the area. It must be because there are more people moving into the area.
The Chief Executive has a Law degree. We'd better make sure we're all above board.
You are hot and I am cold. You are wearing a brown coat. So let's go for a drink.

Discussion
Sometimes this fallacy is used by people who want to prove something but do not know how, so they use any argument and then tack their desired conclusion on to the end. This is something that politicians often do.
This is effective persuasion when the listener does not work through the logic of the argument and is persuaded simply by the fact that some kind of argument is being used (as opposed to the conclusion being given as a simple statement). This can be encouraged by speaking with passion and apparent authority.
Personal Inconsistency: Past words or deeds do not match claim.
  • Ad Hominem Tu Quoque
  • You too
Description
Person A makes a claim. Person B asserts that person A's previous claims or actions are inconsistent with person A's claim. Therefore person A's claim is false.
Thus the present is rubbished by dragging up the past.
Example
You say you are against the war, yet you voted for it.
He wants to re-join the club, yet he left it of his own free will last year.

Discussion
Just because what is said now does not align with the past, it does not necessarily mean that it is wrong (it could be that the past was wrong, or that circumstances have changed). There are many times in the past that the person could have produced contradictions to the single point being made—we are not fully rational beings and do contradict ourselves on many occasions.
Personal inconsistency sends mixed messages and will normally decrease trust. When a person claims that another person is sending mixed messages, they are very close to calling a person a liar. If the accusation stands, then everyone who hears is likely to reduce their trust in the accused person.
Personal inconsistency is quite different from logical inconsistency, which is about inconsistency within the logic of an argument.
Poisoning the Well: Discrediting the person before they speak.
  • Discredit
Description
Discredit the other person before they speak. Or discredit the topic or argument that they may support.
There are many ways of discrediting the person. Call them names. Talk about their lies. Show them to be unworthy. Tell how they are unintelligent, crazy or otherwise undesirable, inferior and not worth listening to, let alone believing.
To discredit the topic or argument, indicate how it is patently absurd, proven to be false or that only fools would support it.
Example
Well, Jane will tell you something else, but then she always lived on the other side of the tracks.
Mike doesn't have a degree, but he does speak nicely, doesn't he.
Only an idiot would consider Didactus to have any useful opinion.
Everybody knows that cold fusion is a proven impossibility. Jack: did you have something to say on this.

Discussion
By discrediting the other person, you are also effectively discrediting anything they say by reducing their authority. If the other person is there, a public attack forces them onto the defensive, socially obliging them to respond first to the attack and hence distracting them from their main argument. If the other person is not there, then they cannot defend themselves.
Personal attack always has its hazards, and other people, especially rescuers, may well leap to their defense.
Post Hoc: X follows Y. Therefore X is caused by Y.
  • Post hoc, ergo propter hoc
Description
If X follows Y, then X is caused by Y.
The sequence of things proves cause.
Example
The man pulled out a gun. A shot was fired. Therefore the man fired the shot.
You used the telephone and then it stopped working. You broke the phone.
I am feeling very unwell. It must have been the meal last night.

Discussion
Just because something follows something else, this is not sufficient evidence to prove true cause and effect. This temporal relationship may simply be coincidence.
Coincidence is often related to superstition—hence saying 'bless you' when someone sneezes (it is assumed that sneezing lays a person open to spiritual attack) or throwing salt over your shoulder when you spill it (it is assumed to cause bad luck otherwise).
Red Herring: Distracting with an irrelevancy.
Description
We are talking about X. Y is mentioned. The conversation changes to Y.
If you want to avoid talking about something, change the subject.
Pick something that will engage the other people. It can be completely off the current track or something related, but not really relevant. Something controversial or anything that arouses their emotions is often a good idea.
Example
What about Christmas? Well, my aunt is coming next week.
This is expensive. Mind you, I heard that we might get a raise soon.
Yes, it is expensive, sir. Is that a scratch? No, it's reflection in the fine paintwork.

Discussion
Red Herrings, if they snag the interest of the other party, can cause the conversation to change direction (and perhaps away from the direction the speaker does not want it to go).
When a word or sentence does not fit in with other words and sentences, the overall semantics are lost and listeners are confused. In that state of confusion they are open to other suggestions.
Reification: Treating a concept as concrete reality.
  • Concretism
  • Hypostatisation
  • Misplaced Concreteness
Description
The reification fallacy occurs where an abstract idea, concept or model is treated as if it were concrete and real.
Example
A computer is like a brain. It can make intuitive leaps as well.
The Boston Matrix tells us that our product range got a cash cow, two question marks and a dog. We hence should remove one question mark and the dog.
Alan is a god amongst men. He will know you better than you know yourself. He will be able to heal you with a single touch.

Discussion
One of the skills of the human species is the ability to think in abstract terms, juggling ideas that help us understand and work with the real world. This is in some ways essential as the world is too complex for us to understand in infinite detail.
We naturally build inner mental models as a way of coping with this outer complexity. We then view the world through the models, treating the model as if it is the world, not just a representation. In this sense, it could be said that everything is a reification.
Where the reification fallacy occurs in an important sense is where the assumption of idea as reality is too far from a better truth. In business, for example, many models are used to understand and describe business situations. When the models are taken too literally, people can end up depending on them and making blind decisions rather than using the models to give one viewpoint that may be considered alongside other evidence.
Reification may be deliberately used in the use of metaphor and other figures of speech. It becomes a fallacy when we forget that the representation is just that: a representation, and not reality. Extended metaphors can easily fail in this way.
Repetition: Repeating something makes it more true.
  • Ad Nauseum
  • Nagging
Description
The more X is repeated, the more true it becomes.
Repeating something, over and over and over (yes, I know, I know), makes it right, true and more certain.
Example
That is bad, bad, bad. Oh, that's so bad.
Dad, I'm thirsty. I want a drink. I really need a drink. Can I have a drink, please.
If I've told you once, I've told you a thousand times...
Listen, everyone, I want to tell you how I have been enlightened!

Discussion
When I am uncertain about something myself, I will seek ratification of my decision through confirmation by other people. This is one reason why people smitten by an ideology will preach it to others—not so much as wanting to see them saved, but to prove by the conversion of other people the validity of their own conversion.
Repeating the same thing makes it more available to other people, which they then may confuse with statistics to assume that this makes it more probable. It also makes the speaker more confident. If I say something repeatedly, it must be because it is true.
Nagging is also a way of wearing people down. Eventually they will agree with you just to get you off their backs. There is also danger in this, as it can drive people away or result in them snapping back at you. They may reduce their commitment as they feel you are trying to control them.
However, it still remains that repeating something does not make it true.
Slippery Slope: Loosely connected statements with ridiculous conclusion.
  • Absurd Extrapolation
Description
The slippery slope is a series of statements that have a superficial connection with one another, and which lead into what is often a rather far-fetched conclusion.
Use it when you are in relatively unsophisticated company where such obvious lack of logic will go unchallenged. In more thoughtful company, you will need a more subtle approach.
Example
If we ban smoking, then people will start taking soft drugs and then move onto hard drugs, and the crime rate will go up and up. We should therefore prevent crime by allowing smoking.
Discussion
The slippery slope is particularly obvious in its lack of real reason, yet it appears surprisingly often. It is often used in emotional situations where careful thought is replaced by an irrational need for illogical proof and justification.
This fallacy is often used by politicians. This is possibly because they tend to be risk-averse, and the slippery slope seems like a good way of pointing out dangers ('It'll all end in tears!').
Social Conformance: Agree with me or be socially isolated.
Description
If you do not agree, then you will be rejected and ostracized by your friends.
Use peer pressure instead of rational argument to get agreement.
Example
If you want to lose all your friends, just keep on saying that.
I don't know if Paul and Peter want to associate with anyone who wanted to go their own way like that.
You know, that's not the sort of thing we do here.

Discussion
Social acceptance is hugely important to most people, especially with those they consider to be friends and peers. The threat of rejection is thus taken very seriously and people will change their position and even their beliefs to sustain their social position.
This fallacy is very common in everyday conversation, either explicitly or implied. It is even read into arguments when it is not actually there.
Strawman: Attack a weak argument used by the other person, making it seem their entire argument.
Description
You have a several arguments for your case. I disprove one of those arguments, therefore the whole case is false.
Rather than attack the strongest argument, go for a weak one that is easier to attack.
Seek to change their real position to that where you can attack it.
Example
Astrology may be unproven, but neither has it been proved to be false.
You said the common man is important, so show me this 'common man'.
You want to spend less on education. Do you really want to cripple this country's future?

Discussion
By picking on a weak part of the argument and making a big deal of it, attention is distracted from the stronger reasons that should be the main part of the discussion.
The basic assumption is that if one small part of an argument can be proved false then, by association, the whole argument is also false.
A weak argument is one made of straw that is easily knocked over. Hence the name 'strawman'.
Style over Substance: An attractive presentation makes it more right.
Description
Presenting something in an attractive way makes it more right.
Dress well. Speak well. Use props, pictures and powerpoint. Use panache, verve, flair and elegance. Wow the crowds with your style and they may not notice that your content is not so hot. Use elaborate language that sounds good and fills up the space you have, covering the fact that you may actually have very little to say.
Example
Good ladies and Gentle men, let me first welcome you to this auspicious occasion...
[Arrive to sound of trumpets] Thank you folks. Well, I don't want to blow my own trumpet, you know...(laughter)
Well, how can I disagree with such a smart young man.

Discussion
Just because something is attractive does not make it right.
If people like you, then they will bond with you and will be unwilling to criticize what you say, or even think ill of you—to do otherwise would cause Cognitive Dissonance.
Undistributed Middle: All A is B. All C is B. Therefore all C is A.
Description
All A is B. All C is B. Therefore all C is A.
B is assumed to cover all items in its category.
Example
All Californians are beautiful. All women are beautiful. Therefore all women are Californian.
All fools act stupid. You acted stupid. Therefore you are a fool.
All elephants are big. Some boys are big. Therefore some boys are elephants.

Discussion
The problem here is that the middle term (that connects the first two statements) is assumed to refer to the same thing—typically all of the members in its category, yet this is seldom true. Thus, in the first example above, neither all Californians nor all women cover all of the beautiful people in the world (some British men are beautiful).
In effect, the 'reasonable' assumption is that the first two statements are of the form A=B and C=B, from which the mathematically sound conclusion is that C=A. Unfortunately, syllogisms deal with sets, not mathematical variables.
Unrepresentative Sample: What is true about any sample is also true about the population.
  • Biased Sample
  • Fallacy of Exclusion
Description
Sample X is taken from Population Y. Conclusion Z is drawn from sample X. It is assumed that Z is also true about Y.
Take a biased or otherwise statistically invalid sample. Analyze the data. Draw conclusions and declare the results significant.
Example
We surveyed homes during the day and found that 66% of the population enjoy soaps.
I asked four people in the street and three liked red. 75% of people like red.
Nine out of ten of cat owners we asked agreed that their cats like KitaKit.

Discussion
Most people believe they are pretty good at making statistical assessments. In fact we are generally pretty poor at it, and there are many traps into which we fall. Taking an unrepresentative sample is one of the most basic of these.
Where a sample is deliberately biased by leaving out data, this is the Fallacy of Exclusion.
Wishful Thinking: A is true because I want it to be true.
  • Appeal to Consequences of a Belief
Description
1. I want something to be true and factual.
2. Therefore it is true and factual.
To think wishfully, just act as if what you want is true and either has happened or is about to happen.
Example
The book I have written is loved by everyone who reads it.
I need a break!

Discussion
Despite the obvious falsehood of this fallacy, it is surprising how often it appears. It is often also surprising how often people do not realize that they are doing it, as subconscious desires appear through assumptions of truth.
People who use wishful thinking often supplement it with emotional states such as aggression or pleading, seeking either to batter others into accepting their assertion or otherwise adopt a child position.
Wishing can actually lead to something becoming true, where the person acts to reduce the dissonance of conflicting reality and wishes.
When faced with the truth, they are very likely to use denial or use some other form of defense.

The ability to identify logical fallacies in the arguments of others—and to avoid them in one’s own arguments—is both valuable and increasingly rare. Fallacious reasoning keeps us from knowing the truth, and the inability to think critically makes us vulnerable to manipulation by those skilled in the art of rhetoric.

WHAT IS A LOGICAL FALLACY?
A logical fallacy is, roughly speaking, an error of reasoning. When someone adopts a position, or tries to persuade someone else to adopt a position, based on a bad piece of reasoning, they commit a fallacy. I say “roughly speaking” because this definition has a few problems, the most important of which are outlined below. Some logical fallacies are more common than others, and so have been named and defined. When people speak of logical fallacies, they often mean to refer to this collection of well-known errors of reasoning, rather than to fallacies in the broader, more technical sense given above.

FORMAL AND INFORMAL FALLACIES
There are several different ways in which fallacies may be categorized. It is possible, for instance, to distinguish between formal fallacies and informal fallacies.

FORMAL FALLCIES (Deductive Fallacies)
Philosophers distinguish between two types of argument: deductive and inductive. For each type of argument, there is a different understanding of what counts as a fallacy.

Deductive arguments are supposed to be water-tight. For a deductive argument to be a good one (to be “valid”), it must be absolutely impossible for both its premises to be true and its conclusion to be false. With a good deductive argument, that simply cannot happen; the truth of the premises entails the truth of the conclusion.

The classic example of a deductively valid argument is:

(1) All men are mortal.
(2) Socrates is a man.

Therefore:

(3) Socrates is mortal.

It is simply not possible that both (1) and (2) are true and (3) is false, so this argument is deductively valid.

Any deductive argument that fails to meet this (very high) standard commits a logical error, and so, technically, is fallacious. This includes many arguments that we would usually accept as good arguments, arguments that make their conclusions highly probable, but not certain. Arguments of this kind, arguments that are not deductively valid, are said to commit a “formal fallacy”.

INFORMAL FALLACIES (Inductive Fallacies)
Inductive arguments need not be as rigorous as deductive arguments in order to be good arguments. Good inductive arguments lend support to their conclusions, but even if their premises are true then that does not establish with 100% certainty that their conclusions are true. Even a good inductive argument with true premises might have a false conclusion; that the argument is a good one and that its premises are true only establishes that its conclusion is probably true.

All inductive arguments, even good ones, are therefore deductively invalid, and so “fallacious” in the strictest sense. The premises of an inductive argument do not, and are not intended to, entail the truth of the argument’s conclusion, and so even the best inductive argument falls short of deductive validity.

Because all inductive arguments are technically invalid, different terminology is needed to distinguish good and bad inductive arguments than is used to distinguish good and bad deductive arguments (else every inductive argument would be given the bad label: “invalid”). The terms most often used to distinguish good and bad inductive arguments are “strong” and “weak”.

An example of a strong inductive argument would be:

(1) Every day to date the law of gravity has held.

Therefore:

(2) The law of gravity will hold tomorrow.

Arguments that fail to meet the standards required of inductive arguments commit fallacies in addition to formal fallacies. It is these “informal fallacies” that are most often described by guides to good thinking, and that are the primary concern of most critical thinking courses and of this site.

LOGICAL AND FACTUAL ERRORS
Arguments consist of premises, inferences, and conclusions. Arguments containing bad inferences, i.e. inferences where the premises do not give adequate support for the conclusion drawn, can certainly be called fallacious. What is less clear is whether arguments containing false premises but which are otherwise fine should be called fallacious.

If a fallacy is an error of reasoning, then strictly speaking such arguments are not fallacious; their reasoning, their logic, is sound. However, many of the traditional fallacies are of just this kind. It is therefore best to define fallacy in a way that includes them: both formal and informal fallacies, and both logical and factual errors.

TAXONOMY OF FALLACIES
Once it has been decided what is to count as a logical fallacy, the question remains as to how the various fallacies are to be categorized. The most common classification of fallacies groups fallacies of relevance, of ambiguity, and of presumption.

Arguments that commit fallacies of relevance rely on premises that are not relevant to the truth of the conclusion. The various irrelevant appeals are all fallacies of relevance, as are ad hominems.

Arguments that commit fallacies of ambiguity, such as equivocation or the straw man fallacy, manipulate language in misleading ways.

Arguments that commit fallacies of presumption contain false premises, and so fail to establish their conclusion. For example, arguments based on a false choice or circular arguments both commit fallacies of presumption.

These categories have to be treated quite loosely. Some fallacies are difficult to place in any category; others belong in two or three. The ‘No True Scotsman’ fallacy, for example, could be classified either as a fallacy of ambiguity (an attempt to switch definitions of “Scotsman”) or as a fallacy of presumption (it begs the question, reinterpreting the evidence to fit its conclusion rather than forming its conclusion on the basis of the evidence).