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Logical fallacies (I) : uses and abuses of logic

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Many times when you're arguing a case in a presentation, you’re going to be confronted with seemingly logical objections from members of your audience. However, often what appears to be a supporting argument for something doesn’t really support it at all. It’s what’s known as a ‘logical fallacy.’ Often these are committed by people unintentionally, because they’re ignorant of how to reason logically or what counts as relevant evidence. Other times people use them deliberately to deceive and confuse. I’ve listed seven of them below, with examples. If you want to use them yourself, feel free; they can be very effective. If, on the other hand, you don’t, at least you’ll be able to identify them and build up an immunity to them.

1. Freak situations
This technique proposes that the freak nature of an extremely unlikely or ‘one-off’ situation are enough to justify rejection of a general rule. When children claim that ‘my friends all do it’, virtually all parents have responded at one time or other with, “Well if your friends all jumped off a cliff / put their hands in the fire / dove under a bus, would you do that as well?”

Almost every generalization could be objected to on the grounds that a freak ‘on-off’ case would render it inapplicable. However, most generalizations carry some king of implicit assumption that it applies ‘all things being equal.’ If all things aren’t equal, e.g. the child’s friends were seized by a collective insanity and jumped off a cliff in a mass suicide, this doesn’t stop the generalization from being right.

When others try to use this with you, don’t argue about the specific merits of their case. Just treat it with the disdain it deserves and simply point out the extreme unlikelihood/insanity of it ever happening. Watching ‘Dragons’ Den’ on TV a couple of nights ago (it’s called 'Shark Tank' in the US), one of the ‘dragons/sharks’ objected, “But if everybody did XYZ, your idea wouldn’t work.” The contestant just looked him confidently in the eye and said, “But that would be insane.” All the other dragons just laughed, and his objection simply crashed.

2. Old is Better (Argumentum ad antiquitam)
This technique supposes that something is good or better purely because it’s old/traditional/ always been done like that. Again, parents use it when they say things like, “I worked my way through college/didn’t get a car until I was 20, so did my father and so did his father before him. If it was good enough for us, it’s good enough for you!”

Gun owners in the US use it when they claim that the right to bear arms and is enshrined in the Constitution and is nearly 250 years old.

In business, opponents of your proposal will say, “We’ve always done it this way” or “In this company we have certain ways of doing things, and one of them is . . .” or “If it ain’t broke, don’t fix it.”

To automatically equate older with better is ridiculous. Progress is made by replacing the old with the new. Mankind has held on to various views and practices for hundreds or thousands of years (e.g. the sun revolves around the earth, the earth is flat, a human sacrifice will appease the rain god, people with alternative religious views should be burned at the stake) but it didn’t make any of them right.

If someone uses it against you, use gentle ridicule. Say “That’s just what the Romans  called argumentum ad antiquitam – automatically assuming something’s good because it’s old (NB: accusing someone of something that has a Latin name is always impressive; it sounds like they have a contagious disease).

Follow it with:

You’re like the construction workers’ foreman who objected to the use of a mechanical digger by saying ‘If you hadn’t bought that machine, 10 men could do that job with shovels.’ The site owner simply said, ‘And if I hadn’t bought the shovels, 100 could do it with teaspoons.’”


“We can’t block progress and nor should we want to. President Eisenhower once said, 'It is neither wise nor brave for a man to lie down on the tracks of history and wait for the train of the future to run over him.''”

3. Either/or (Bifurcation)
This is presenting everything in black & white, as if there were only two alternatives, as in President Bush’s “You are either with us or against us.” It ignores the fact that there are usually many shades of grey (at least 50, according to popular literature!) and a range of options.

A businessperson might say, "There are two types of company in this world; those that constantly reinvent themselves and survive, and those that die. Which do we want to be?” This ignores the millions of companies that change occasionally or gradually and do very well, thank you.

If you want to use it yourself, introduce it with the words, “It seems to me there are only two possible courses of action. We can …….”

If someone uses it against you, don’t get drawn into an either/or argument. Simply accuse them of bifurcating and point out that there are, in fact, a whole variety of options they haven’t considered.

4. Going in circles (Circulus in probando)
This uses a fact in support of an argument which is authenticated by the very argument it supports. For example:

Why should I believe in God?”
“Because the Bible/Koran/Talmud tells us he exists.”
"And why should I believe the Bible/Koran/Talmud?”
“Because it’s the word of God

I wouldn’t ask you to do this if I didn’t believe you could do it.”
“And how do I know you really think I can do it?”
“Would I ask you if I didn’t?”

"Companies are switching en masse from traditional media to advertising on Facebook and other social media sites."
"How do we know it's the right thing to do?"
"That many companies wouldn't be doing it unless it is."

"Why do you wear a hair from a virgin's head in a locket?"
"It protects me from lions."
"But there aren't any lions in Illinois."
"That shows how effective it is."

"If X wasn't immoral, it wouldn't be illegal."

5. Casual Connections (Cum hoc ergo propter hoc)
Sometimes people take two completely unrelated things that occurred together and link them, completely ignoring coincidence or outside factors that have influenced events. For example, “Jesus wore sandals. I wear sandals. Therefore I must be Jesus.”

A statistician once studied the handwriting of 7-12 year olds and noticed a definite correlation between handwriting neatness and foot size. The bigger the child’s feet, the neater his/her handwriting, with a 99% probability rate. This was published as a serious theory until someone pointed out that older children tended to be neater writers, and guess what? They also had bigger feet.

6. Every Schoolboy Knows
This technique is deliberately used to squash discussion by making people feel stupid or ignorant if they object. The recipients are expected to assent not because they really agree but out of shame and fear. For example: “The scientists are virtually screaming from the rooftops now. The debate is over! There's no longer any debate in the scientific community about this” – Al Gore.

In reality, the use of this is usually a good indicator that what’s being said is not universally accepted, so become extremely cautious when you hear it. Nobody feels the need to use it when a fact really is undisputable and isn’t still being debated, e.g. “It’s generally accepted by mathematicians around the world that 2 plus 2 equals 4,” or “Thousands of eminent historians agree that the battle of Waterloo was fought in 1815. The debate is over.”  If something really is an undisputed fact, the technique doesn’t need to be used.

A simple use of the technique is just using the word ‘obviously,’ as in “Obviously, we have to do something about this.”

7. Moderation (Argumentum ad temperantium)
If two people were arguing and one said ‘2+2=4’ while the other said ‘2+2=6’, somebody would probably say, “Let’s be reasonable about this and split the difference. Let’s all agree that 2+2=5.”

When Iraq first invaded Kuwait and an international coalition opposed it, there was probably somebody in the United Nations who said, “Saddam Hussein wants all of Kuwait, the rest of the world wants him to have none. Let’s all be reasonable and agree he can have half.”

Much of negotiation is based on this, as in “The unions are threatening to strike unless they get 6%, management have offered 2%. Why don’t we avoid the unpleasantness of a strike and shake hands on 4%?

An appeal to reason is very powerful because it appeals to a very deep rooted instinct in most people that moderation is ‘fair.’ Who wants to be seen as being unreasonable? But while it seems ‘fair’, taking the average of two views cannot be correct if one of those views is right and the other wrong.


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