This article is from the Atheism FAQ, by mathew firstname.lastname@example.org with numerous contributions by others.
One of the simplest fallacies is to rely on anecdotal evidence. For
"Violent crime is on the increase because you hear a lot more about
it on the news these days."
It's quite valid to use personal experience to illustrate a point; but
such anecdotes don't really prove anything to anyone. Your friend may
say he met Elvis in the supermarket, but those who haven't had the
same experience will require more than your friend's anecdotal
evidence to convince them.
Argumentum ad baculum / Appeal to force
An Appeal to Force happens when someone resorts to force (or the
threat of force) to try and push others to accept a conclusion. This
fallacy is often used by politicians, and can be summarized as "might
makes right". The threat doesn't have to come directly from the person
arguing. For example:
"... Thus there is ample proof of the truth of the Bible. All those
who refuse to accept that truth will burn in Hell."
"... In any case, I know your phone number and I know where you
live. Have I mentioned I am licensed to carry concealed weapons?"
Argumentum ad hominem
Argumentum ad Hominem literally means "argument directed at the man".
There are two types, abusive and circumstantial.
If you argue against some assertion by attacking the person who made
the assertion, then you have committed the abusive form of argumentum
ad hominem. A personal attack isn't a valid argument, because the
truth of an assertion doesn't depend on the virtues of the person
asserting it. For example:
"Atheism is an evil philosophy. It is practised by Communists and
Sometimes in a court of law doubt is cast on the testimony of a
witness. For example, the prosecution might show that the witness is a
known perjurer. This is a valid way of reducing the credibility of the
testimony given by the witness, and not Argumentum ad Hominem.
However, it doesn't demonstrate that the witness's testimony is false.
If you argue that someone should accept the truth of an assertion
because of that person's particular circumstances, then you have
committed the circumstantial form of argumentum ad hominem. For
"It is perfectly acceptable to kill animals for food. How can you
argue otherwise when you're quite happy to wear leather shoes?"
This is an abusive charge of inconsistency, used as an excuse for
dismissing the opponent's argument. The fallacy can also be used as a
means of rejecting a particular conclusion. For example:
"Of course you would argue that positive discrimination is a bad
thing. You're white."
This particular form of Argumentum ad Hominem, when you allege that
someone is rationalizing a conclusion for selfish reasons, is also
known as "poisoning the well".
Argumentum ad ignorantiam
Argumentum ad ignorantiam means "argument from ignorance". The fallacy
occurs when it's argued that something must be true, simply because it
hasn't been proved false. Or, equivalently, when it is argued that
something must be false because it hasn't been proved true.
(Note that this isn't the same as assuming that something is false
until it has been proved true; that's a basic scientific principle.)
Here are a couple of examples:
"Of course the Bible is true. Nobody can prove otherwise."
"Of course telepathy and other psychic phenomena do not exist.
Nobody has shown any proof that they are real."
Note that this fallacy doesn't apply in a court of law, where you're
generally assumed innocent until proven guilty.
Also, in scientific investigation if it is known that an event would
produce certain evidence of its having occurred, the absence of such
evidence can validly be used to infer that the event didn't occur.
"A flood as described in the Bible would require an enormous volume
of water to be present on the earth. The earth does not have a
tenth as much water, even if we count that which is frozen into ice
at the poles. Therefore no such flood occurred."
In science, we can validly assume from lack of evidence that something
hasn't occurred. We cannot conclude with certainty that it hasn't
Of course, the history of science is full of logically valid bad
predictions. In 1893, the Royal Academy of Science were convinced by
Sir Robert Ball that communication with the planet Mars was a physical
impossibility, because it would require a flag as large as Ireland,
which it would be impossible to wave. (Source: Fortean Times Number
See also Shifting the Burden of Proof.