I've seen them in girl-girl porn flicks...
Sorry, I know it's unoriginal but I'm ill in bed and bored.
Most common fallacies I come across... (Hopefully I do the descriptions justice.)
1. By far the most common is ad hominem. The most obvious example of this is name calling... or think of practically any political TV ad in the USA.
2. No True Scotsman. Example: A true christian would never become an atheist therefore no atheist ever was a true christian.
3. Begging the question/circular reasoning. Example: The bible is right because the bible says so or circular reasoning works because circular reasoning works.
4. Burden of Proof. This is when the person making a positive claim argues that those making the negative claim have to prove that their positive claim is false. Example: I believe god exists. Where's your proof? You can't prove that god doesn't exist.
5. Straw Man. This is when a person misrepresents a position to their benefit usually to make their opposition look weak. Example: Person A argues in favor of universal health care. Person B says that universal health care is socialism. Person B says socialism is bad therefore Person A's argument for universal health care has been defeated.
A dichotomy simply means the division of something into two. A false dichotomy, ergo, is any such division which is fallacious.
It is something that politicians often employ as a rhetorical device. For example, in phrases such as: "You're either with us, or you're against us."
On the face of it, a rational person might be tempted to respond "well I'm not really against you, so I guess I should just come out and support you." If you think about this a little further, however, it becomes obvious that the implied dichotomy isn't actually true.
It is perfectly possible that you are neither for someone nor against them. You may be apathetic or undecided. You may agree with some of their ideas and policies, but not with others. You may disagree with them on just about everything, but still not be "against" them in the sense of wanting to actively oppose them. And so on.
Now for My Top 5 Fallacies. I hear hundreds of them every day, and some of them really wind me up. These seem to be some of the most common:
1) Argumentum ad populum
Simply put, this is an appeal to the majority: "P is true because the majority of people agree that P is true."
How often have we all been infuriated by people who try to prove their position, either by getting their friends to agree with them, or by simply saying "everybody knows that..."
Of course, this form of argument is total nonsense. No assertion is more likely to be true simply because a large number of people believe that it's true.
Before Copernicus, the vast majority of people believed in a geocentric universe. But this widespread belief didn't mean that the geocentric model had to be accurate, and of course it wasn't.
2) Appeal to authority
"P is true because Mary says that P is true. And Mary is a highly respected individual."
An appeal to authority may be permissible, under certain circumstances. For example, you wish to put forward an argument concerning something scientific, but you yourself are not a very good scientist. In this instance, appealing to the authority of, say, Einstein or Hawking may not be the worst move you can possibly make.
However, even in this instance, you're assertion is still not necessarily true simply because Einstein or Hawking have said that it is true. More importantly, I hear this type of appeal from many people on a regular basis, and more often than not it is employed in a totally irrelevant way.
"This must be true because celebrity X said that it is true/because it says so in my favourite magazine."
As any rational person must recognise, even the most respected celebrities can believe in all kinds of crazy nonsense, and most forms of popular jounalism don't concern themselves too much with what constitutes sound, reliable evidence.
Essentially, employing this sort of argument because you have no evidence for your position is akin to the petulant schoolchild who insists that "it's true, because my mummy says so!"
3) Wishful thinking
"If P were true, then this would be more pleasant than if P were not true. Therefore, we should believe P."
I really don't think that it needs to be explained why this is such a terrible argument. But still, it seems to be one that a lot of people are convinced by.
The religious variety of this is one that everyone's probably most familiar with; we should believe in a god and an afterlife because it's more pleasant (or less scary) than the atheists' alternative.
It applies equally to belief in psychics, mediums, astrology, alternative medicine and practically any form of pseudoscience or superstition that you can think of.
It's true because you want it to be true? Seriously, what drivel!
4) Appeal to tradition
"Chinese folk medicine must be effective, because it's been around for centuries."
"This must be a sound method, because it's the method that our society has always used before."
"The Christian religion must be true, because people have believed in it for such a long time."
Need I go on? Of course, there are many good ideas and practices that have been around for a very long time. But there are many bad ones also.
Centuries ago, it was traditional for many white people to keep black people as slaves. Until recent years, fox hunting with dogs was a traditional sport in England. Before the feminist movement, it was traditional for women to be considered subordinate to men. Just because these things were "traditional" did this necessarily make them good or right?
Some very good ideas may indeed have endured for centuries; but so too have idiots, liars and charlatans.
5) The No true Scotsman fallacy
This one is currently one of my favourites. It's essentially a subtle way of disregarding any counter-examples to a particular assertion.
"Islam is a peaceful religion. No true Muslim would ever murder people in the name of Allah."
Unfortunately, a great many Muslims do exactly that. While it's true that many Muslims are perfectly kind and peaceful individuals, what basis can they possibly have for saying that their own version of Islam is any more valid than that of the murderers?
These are just five examples of fallacies that I tend to come across quite regularly. There are many others I could mention, but these are the ones that currently come to mind and probably the ones that I encounter the most often.
Anyone who told you that, Fred, is creating a false dichotomy :) As we all know there are plenty of things that people can believe in that have nothing to do with gods.
Which would usually be followed by a No True Scotsman fallacy by saying that "no REAL atheist believes in that stuff"...
People who tell you this stuff are just plain wrong - for many of the same reasons that they berate a religious person.
Becca & John gave most of the more common examples you're likely to see. There is a couple more that I come across on a semi regular basis:
Equivocation: Using a word to mean different things depending on how one wishes to prop up one's argument. "All heavy things have a great mass; this is heavy fog; therefore this fog has a great mass." -example from Wikipedia
Special Pleading: Claiming that certain properties do not apply to your subject without demonstrable reason. "Everything had a beginning. Except God" is special pleading unless you can demonstrate how God sidesteps the condition of having a beginning.