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Logical fallacies are errors that occur in arguments. In logic, an argument is the giving of reasons (called premises) to support some claim (called the conclusion). There are many ways to classify logical fallacies. I prefer listing the conditions for a good or cogent argument and then classifying logical fallacies according to the failure to meet these conditions.
Every argument makes some assumptions. A cogent argument makes only warranted assumptions, i.e., its assumptions are not questionable or false. So, fallacies of assumption make up one type of logical fallacy. One of the most common fallacies of assumption is called begging the question. Here the arguer assumes what he should be proving. Most arguments for psi commit this fallacy. For example, many believers in psi point to the ganzfeld experiments as proof of paranormal activity. They note that a .25 success rate is predicted by chance but Honorton had some success rates of .34. One defender of psi claims that the odds of getting 34% correct in these experiments was a million billion to one. That may be true but one is begging the question to ascribe the amazing success rate to paranormal powers. It could be evidence of psychic activity but there might be some other explanation as well. The amazing statistic doesn't prove what caused it. The fact that the experiment is trying to find proof of psi isn't relevant. If someone else did the same experiment but claimed to be trying to find proof that angels, dark matter, or aliens were communicating directly to some minds, that would not be relevant to what was actually the cause of the amazing statistic. The experimenters are simply assuming that any amazing stat they get is due to something paranormal.
Another common--and fatal--fallacy of assumption is the false dilemma, whereby one restricts consideration of reasonable alternatives.
Not all fallacies of assumption are fatal. Some cogent arguments might make one or two questionable or false assumptions, but still have enough good evidence to support their conclusions. Some, like the gambler's fallacy, are fatal, however.
Another quality of a cogent argument is that the premises are relevant to supporting their conclusions. Providing irrelevant reasons for your conclusion need not be fatal, either, provided you have sufficient relevant evidence to support your conclusion. However, if all the reasons you give to support of your conclusion are irrelevant then your reasoning is said to be a non sequitur. The divine fallacy is a type of non sequitur.
One of the more common fallacies of relevance is the ad hominem, an attack on the one making the argument rather than an attack on the argument. One of the most frequent types of ad hominem attack is to attack the person's motives rather than his evidence. For example, when an opponent refuses to agree with some point that is essential to your argument, you call him an "antitheist" or "obtuse."
A third quality of a cogent argument is sometimes called the completeness requirement: A cogent argument should not omit relevant evidence. Selective thinking is the basis for most beliefs in the psychic powers of so-called mind readers and mediums. It is also the basis for many, if not most, occult and pseudoscientific beliefs. Selective thinking is essential to the arguments of defenders of untested and unproven remedies. Suppressing or omitting relevant evidence is obviously not fatal to the persuasiveness of an argument, but it is fatal to its cogency. The regressive fallacy is an example of a fallacy of omission. The false dilemma is also a fallacy of omission.
A fourth quality of a cogent argument is fairness. A cogent argument doesn't distort evidence nor does it exaggerate or undervalue the strength of specific data. The straw man fallacy violates the principle of fairness.
A fifth quality of cogent reasoning is clarity. Some fallacies are due to ambiguity, such as the fallacy of equivocation: shifting the meaning of a key expression in an argument. For example, the following argument uses 'accident' first in the sense of 'not created' and then in the sense of 'chance event.'
Since you don't believe you were created by a god then you must believe you are just an accident. Therefore, all your thoughts and actions are accidents, including your disbelief in any god.
Finally, a cogent argument provides a sufficient quantity of evidence to support its conclusion. Failure to provide sufficient evidence is to commit the fallacy of hasty conclusion. One type of hasty conclusion that occurs quite frequently in the production of superstitious beliefs and beliefs in the paranormal is the post hoc fallacy.
Some fallacies may be classified in more than one way, e.g., the pragmatic fallacy, which at times seems to be due to vagueness and at times due to insufficient evidence.
The critical thinker must supplement the study of logical fallacies with lessons from the social sciences on such topics as
James Alcock reminds us that “The true critical thinker accepts what few people ever accept -- that one cannot routinely trust perceptions and memories” (“The Belief Engine”). The unhappy truth is that humans are not truth-seeking missiles. In addition to understanding logical fallacies, we must also understand why we are prone to them.
There are literally hundreds of logical fallacies. For a good general introduction to fallacies I recommend Attacking Faulty Reasoning: A Practical Guide to Fallacy-Free Arguments by T. Edward Damer or Asking the Right Questions: A Guide to Critical Thinking by M. Neil Browne and Stuart M. Keeley.
There are some on-line sites that focus on fallacies. I refer the reader to them without comment:
The Nizcor Project (Dr. Michael C. Labossiere)
here for a
complete list of the SD critical thinking mini-lessons