Argumentation and Logic, part 2: Fallacies

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After my last pretty heavy-hitting post, this one should be a bit easier to follow.  Identifying a fallacy requires some subtlety, but can become second-nature after some practice.  That being said, one of the real pitfalls of catching fallacies is that one can obsess over them, seeming to find them everywhere.  This is often due to misusing the new-found "power," identifying fallacies that aren't there in the first place.  Today we find ourselves surrounded by sloppy skeptics of just about every subject, determined to slap "fallacy" on the side of any argument.  Because of this, some care is required.

There are both formal and informal fallacies.  A formal fallacy is an argument that is invalid. Interestingly, this says nothing of the premises or the conclusion of the argument; it is only the structure of the argument that is at fault.  An argument may be fundamentally flawed and yet the conclusion be correct.  Formal fallacies are not limited to deductive arguments.  Inductive arguments that misapply the principles of probability, statistics, or causality are also considered formal fallacies.  An informal fallacy, on the other hand, is an argument that can suffer from any number of shortcomings that render the argument unpersuasive; the error in the argument is not a flaw in the logic but in something else, such as the plausibility of the premise(s).  In other words, an informal fallacy is found in the argument's contents, rather than in the form of the argument.

Let's look at some examples of formal and informal fallacies.

Formal Fallacies

One type of formal fallacy is called a propositional fallacy, which is an error in reasoning when combining multiple propositions.  There are two very common forms of propositional fallacies, known as affirming the consequent and denying the antecedent.  They both result from misusing if-then arguments (known as conditionals).  There are two valid forms of this argument and two invalid forms.  The first valid one follows the form: If A, then B.  A.  Therefore, B.  For example,

1.  If that device is an iPod, then it was made by Apple.
2.  That device is an iPod.
3.  Therefore, it was made by Apple.

The Latin name for this form of argument is modus ponens.  The second follows the form (now follow closely here): If A, then B.  Not B.  Therefore, not A.  For example,

1.  If the car was made by Subaru, it is reliable.
2.  It is not reliable.
3.  Therefore, it was not made by Subaru.

The Latin name for this form of argument is modus tollensModus tollens takes some getting used to, as it seems to be false at first glance.  In fact, I recently found an apologetics blog that listed it as a fallacy!  Affirming the consequent, on the other hand, affirms B (the consequent) is true, and concludes that A is true.  Here's an example:

1.  If the car was made by Subaru, it is reliable.
2.  It is reliable.
3.  Therefore, it is made by Subaru.

While Subaru may make reliable cars, they are not the only manufacturer to do so.  It could be the case that the car is a Honda or a Toyota, or even a one-in-a-million Land Rover.

The second fallacy using a conditional is known as denying the antecedent, which denies A (the antecedent) is true, and concludes that B is not true.

1.  If that device is an iPod, then it was made by Apple.
2.  It is not an iPod.
3.  Therefore, it was not made by Apple.

It could be any number of other Apple products, such as an iPhone.  Other formal fallacies include the following:

Fallacy of necessity - this fallacy is not often mentioned in lists of fallacies, but it is an important one.  Here is an example:

1.  Bachelors are necessarily unmarried.
2.  John is a bachelor.
3.  Therefore, John cannot marry.

Clearly, John could simply cease being a bachelor by getting married.   C assumes B is always true.  Simply change B, and the tautology of A no longer holds.  I often hear it claimed that "atheists never honestly look at the evidence, and so never see the truth."  A case could be made that someone like C.S. Lewis honestly looked at the evidence, and became a Christian as a result.  Using C.S. Lewis as an example, the statement implicitly argues:

1.  Atheists never honestly look at the evidence
2.  C.S. Lewis is an atheist.
3.  C.S. Lewis cannot become a Christian.

It is a well known fact that C.S. Lewis did become a Christian.  Likewise, it is often argued that Christians never listen to reason, and so are never convinced their beliefs are not true.  A counterexample for this is someone like Dan Barker.

Appeal to probability - this takes something for granted simply because it is probable (or simply possible)

1.  The Indians haven't been doing well lately, and the odds are they'll lose given the pitcher they're facing.
2.  Therefore, the Indians will lose.

There are many other formal fallacies; for example, the penguin from my last post suffered from a syllogistic fallacy.

Informal Fallacies

There are an incredible number of informal fallacies, far too many to discuss in a single post.  That said, some of the fallacies fall into two categories: faulty generalizations and red herrings.

Faulty Generalizations

These arguments simply reach conclusions from weak premises.

No True Scotsman - named after the famous example by Antony Flew, this is an ad hoc attempt to preserve a poor assertion.  Here is what Antony Flew wrote:
Imagine Hamish McDonald, a Scotsman, sitting down with his Glasgow Morning Herald and seeing an article about how the "Brighton Sex Maniac Strikes Again". Hamish is shocked and declares that "No Scotsman would do such a thing". The next day he sits down to read his Glasgow Morning Herald again; and, this time, finds an article about an Aberdeen man whose brutal actions make the Brighton sex maniac seem almost gentlemanly. This fact shows that Hamish was wrong in his opinion but is he going to admit this? Not likely. This time he says, "No true Scotsman would do such a thing".1
Cherry picking - this is when someone selects only certain evidence that supports his conclusion, while ignoring other evidence that may destabilize his case.

Red Herrings

Tradition has it that the term "red herring" comes from the days of fox hunting, where dogs being trained to follow a scent were often tested by dragging a red herring - a fish - across the scent trail in order to distract them.  So, these are arguments that distract one from the issues at hand.

Ad hominem - simply put, attacking the person rather than the argument.  "That guy is wrong because he's an idiot."  This term has been abused somewhat, so that it often is used to include any sort of personal attack, whether to invalidate an argument or no.

Appeal to authority - "Stephen Hawking said philosophy is dead, and he's a genius; therefore, philosophy is dead."  This fallacy is often abused.  If a community of experts in a given field reach a consensus on a certain issue, it is likely their opinion can be trusted; in such a case an appeal to authority may be warranted.  However, if an expert (or group of experts) is speaking on a subject matter outside of her field, she may be speaking out of ignorance.  It is also possible that she is an expert on the subject, but there is little consensus among other experts on the issue.  In either case, an appeal to authority would be fallacious.

Argument from silence - this type of argument claims that something is false because there is no positive evidence for it.  At the very least, unless there is compelling evidence against it, one should withhold judgment.

Genetic fallacy - the claim that an argument is false because of some part of the background of the person who is making the argument.  "You're just saying that because you grew up in a conservative Christian home!"

Chronological snobbery - this term was coined by C.S. Lewis and Own Barfield.  Lewis in Surprised by Joy defines it best: "the uncritical acceptance of the intellectual climate common to our own age and the assumption that whatever has gone out of date is on that account discredited."  I mainly included this fallacy because the name is so exquisite. 

A Final Difficulty with Arguments

In closing out my two posts on logic and argumentation, I thought it would be good to end with a danger that the philosopher Alvin Plantinga pointed out in a lecture.  In presenting an argument, it is possible that opponents will discover some error in the argument, such as an invalid inference, or a premise that is weak.  However, it may be the case that the argument is strong, both in terms of validity and the strength of the premises and conclusion, yet a person may reject it.  To paraphrase Plantinga, when confronted with an argument a person sees to be valid for a conclusion he strongly disbelieves from premises he knows to be true, he may give up some of those premises.  In so doing, he moves to a position of further ignorance as a result of the argument, rather than into greater knowledge.

This is a danger for anyone, especially when the stakes are high with respect to the issue at hand, such as the existence or nonexistence of God.  No one is exempt from this possible bias.  In posts to follow, we may see some of the effects on both sides of the "God debate."

1.  Flew, Antony (1975).  Thinking About Thinking: Do I Sincerely Want to Be Right?, London: Collins Fontana.
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