Fallacies
A fallacious argument is one that suffers from faulty reasoning. Many forms of faulty reasoning have been identified; following is a list of some of the most common forms of fallacy.
Ad Hominem
The Latin ad hominem means directed at the person. An attempt to persuade through ad hominem comments entails attacking the person making the claim rather than attacking the claim itself.
In ad hominem attacks, the pieces of information supplied may be of interest, but they do not constitute arguments.
Straw Man Argument
A common practice in argument is to ascribe to one's opponent an extreme view that in fact one's opponent has never put forward, and then suggest that, in knocking down the extreme "straw man" argument, you have won.
Below, match the straw-man argument with its refutation
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Neither the opponents nor the supporters of capital punishment make the suggested claims—but often people are able to get away with this sort of sleight of hand in the midst of an argument.
Begging the Question
To beg the question is to assume the conclusion that you are attempting to demonstrate.
Example:
The situation in Afghanistan remains turbulent, and in this context it is vital that we support good government. Clearly we have an obligation to support the present Afghan government: the only question is how this should best be done.
By moving straight to a conclusion that we are obliged to support the present government, the arguer here avoids (or begs) the question of whether or not the current government of Afghanistan is indeed providing good government.
Formally Invalid Arguments
If the formal structure of one's argument is not valid, then the argument is fallacious.
Two common forms of formally invalid arguments are those which deny the antecedent, and those which affirm the consequent.
Example of denying the antecedent:
If water starts dripping from this ceiling during a rainstorm, then you can be sure there is a problem with your roof. No water has dripped from the ceiling during a rainstorm. Therefore, there is no problem with the roof.
In this case it is certainly true that water dripping from the ceiling is a reliable sign of there being some problem with the roof. But of course there can be a problem with the roof even without there being such a visible sign; very frequently, roof leaks result in water saturating rafters and seeping down inside walls without there being any drips from the ceiling.
The if ... part of an argument such as this is known as the antecedent. And, as the example shows, denying the antecedent has no argumentative force.
Example of affirming the consequent:
If a lake is very seriously affected by acid rain, no fish can survive in it. This lake has no fish living in it whatsoever. This lake must be seriously affected by acid rain.
It is entirely true that lakes seriously affected by acid rain cannot support any aquatic life. But that is not the only possible cause for the disappearance of aquatic life from a lake. If, for example, a company had been using the lake as a toxic waste dump, that would also have the effect of killing all the fish. (Again, the distinction between some and all is crucial; some lifeless lakes got that way because of acid rain, but not all). The then ... part of an argument is called the consequent. As the example shows, affirming the consequent does nothing to prove the argument.
Slippery Slope Arguments
The fallacy of the slippery slope argument is the suggestion that one development in a certain direction will inevitably lead to further developments in the same direction or down the same slope.
Example:
The idea of people being required to carry identity cards may seem innocuous enough, but in fact it should be resolutely opposed. If we allow the government to force us to carry identity cards, pretty soon they'll be keeping track of all our movements with video cameras, and placing all sorts of restrictions on our privacy. We have to stop these government intrusions into our lives!
This argument says nothing about the issue of identity cards per se. It is entirely based on the premise of one move by the government being followed by other, more drastic moves. Sometimes, of course, developments are part of long-term trends, but sometimes they are not part of any trend—or may represent the furthest extent of a particular trend. Certainly there is no inevitability about any particular move in one direction being followed by subsequent moves in the same direction.
False Dichotomy
A dichotomy occurs when things or ideas are split into two distinct alternatives. An argument that tries to insist on two and only two alternatives when in fact three or more possibilities exist (or gradations among possibilities exist) is one that poses a false dichotomy.
Example:
A. There should be laws prohibiting people from inciting hate against those of other races or religions.
B. So you're against freedom of speech? Without freedom of speech we wouldn't have a democracy!
A. No, I support freedom of speech—but with certain limits to prevent the most harmful extreme views from being promulgated. It sounds to me as if you are not willing to do anything to combat bigotry and racism.
B. Not at all. But I believe that people speaking out freely against freely expressed bigoted or racist opinions will combat them more effectively than government attempts to prohibit them.
In this case, both arguers pose false dichotomies in their characterizations of the other's viewpoint—whereas in fact both hold nuanced views.
Missing or Unacknowledged Premises
Many real-life arguments take shortcuts and do not spell out all the underlying premises. In many cases this does not in fact damage the argument, but sometimes a missing or unacknowledged premise is the key to finding a fatal weakness in an argument.
Example:
Many have suggested that the presence of extreme poverty and oppression in the world makes it more likely that terrorism will take root. But that just doesn't square with the facts; the vast majority of the September 11th terrorists and the Al Qaeda leadership did not come from backgrounds of extreme hardship. Some, including Osama bin Laden, were among the most privileged members of Saudi Arabian society.
The missing or unacknowledged premise in this argument is that people will always struggle only on behalf of those from their own nationality or social class. In fact, however, history is filled with examples of individuals from one nation or social class who became so involved with the plight of another that they devote all their energies to fighting for change.
An example like this illustrates just how readily our perceptions of argument are influenced by emotion and ideology. In this case, an entirely appropriate sense of anger and revulsion at the terrorist acts makes it difficult for us to imagine that the terrorists might consider themselves as acting altruistically on behalf of the poor and the oppressed. And maybe most of them do not in fact think of themselves in this way. The only point here is that the fact of someone coming from a privileged background does not in itself preclude the possibility that such a person will act in a way he or she perceives as benefiting the less privileged.
Post Hoc, Propter Hoc
The fallacy here is to imagine that if one thing happens after the other, then it will have happened because of the other: post hoc, propter hoc .
Example:
The decline of frog populations throughout the world started to happen just after the thinning of the ozone layer; there has to be a connection!
But there doesn't have to be a connection—as becomes plain if we substitute a different event in the same logical structure:
The decline of frog populations throughout the world started to happen just after the Montreal Canadiens stopped winning the Stanley Cup with any degree of frequency; there has to be a connection!
In the first case damage from ultraviolet radiation is indeed one possible cause for the decline of frog populations—but scientists are still weighing the evidence, and they are far from certain if it is one of several contributing causes, the primary cause, or simply an unrelated event.
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Material from The Broadview Guide to Writing.