APPENDIX: Common Logical Fallacies

Composition

Note that this fallacy moves from the part to the whole by assuming that because something is true about a part of a set it must be true about the whole of the set.

Division

A fallacy of division is an informal fallacy that occurs when one reasons that something that is true for a whole must also be true of all or some of its parts.  This is the opposite of the fallacy of composition.

• The United States is the richest country in the world. Therefore, everyone in the United States must be rich and live well.
• Women in the United States are paid less than men. Therefore, my mom must make less money than my dad.
• Football is the most popular sport in America.  Sally is an American, so she must love football.

Note that this fallacy moves from the whole to the part by assuming that because something is true about an entire set it must be true about each particular in the set.

Appeal to Pity (argumentum ad misericordiam)

Turning this on its head, an appeal to pity tries to win acceptance by pointing out the unfortunate consequences that will otherwise fall upon the speaker and others, for whom we would then feel sorry.

• I am a single parent, solely responsible for the financial support of my children.
• If you give me this traffic ticket, I will lose my license and be unable to drive to work.
• If I cannot work, my children and I will become homeless and may starve to death.
• Therefore, you should not give me this traffic ticket.

Again, the conclusion may be false (that is, perhaps I should be given the ticket) even if the premises are all true, so the argument is fallacious.

Appeal to Emotion (argumentum ad populum)

In a more general fashion, the appeal to emotion relies upon emotively charged language to arouse strong feelings that may lead an audience to accept its conclusion:

• As all clear-thinking residents of our fine state have already realized, the Governor’s plan for financing public education is nothing but the bloody-fanged wolf of socialism cleverly disguised in the harmless sheep’s clothing of concern for children.
• Therefore, the Governor’s plan is bad public policy.

The problem here is that although the flowery language of the premise might arouse strong feelings in many members of its intended audience, the widespread occurrence of those feelings has nothing to do with the truth of the conclusion.

Appeal to Authority (argumentum ad verecundiam)

In an appeal to authority, the opinion of someone famous or accomplished in another area of expertise is supposed to guarantee the truth of a conclusion. Thus, for example:

• Federal Reserve Chair Alan Greenspan believes that spiders are insects.
• Therefore, spiders are insects.

As a pattern of reasoning, this is clearly mistaken: no proposition must be true because some individual (however talented or successful) happens to believe it. Even in areas where they have some special knowledge or skill, expert authorities could be mistaken; we may accept their testimony as inductive evidence but never as deductive proof of the truth of a conclusion. Personality is irrelevant to the truth.

Ad Hominem Argument

The mirror image of the appeal to authority is the ad hominem argument, in which we are encouraged to reject a proposition because it is the stated opinion of someone regarded as disreputable in some way. This can happen in several different ways, but all involve the claim that the proposition must be false because of who believes it to be true:

• Harold maintains that the legal age for drinking beer should be 18 instead of 21.
• But we all know that Harold . . .
• . . . dresses funny and smells bad. or
• . . . is 19 years old and would like to drink legally or
• . . . believes that the legal age for voting should be 21, not 18 or
• . . . doesn’t understand the law any better than the rest of us.
• Therefore, the legal age for drinking beer should be 21 instead of 18.

Two common forms of Ad Hominum are Ad Hominum Abusive and Ad Hominum Circumstantial.

Ad Hominum Abusive – This is where the person is directly attacked for her character, her habits, and even her gender (i.e. This is why a woman shouldn’t do a man’s job.)

Ad Hominem – Circumstantial – Usually this argument attacks a position by appealing to the vested interests of the person who holds the position. Examples: “You can hardly convince me that increases in the military budget are desirable when I happen to know that you work in a munitions factory.” or “Of course, you believe that! You’re a Republican.”

In any of its varieties, the ad hominem fallacy asks us to adopt a position on the truth of a conclusion for no better reason than that someone believes its opposite. But a proposition that a person believes can nevertheless be true (and the intended conclusion false) even if the person is unsavory or has a stake in the issue or holds inconsistent beliefs or shares a common flaw with us. Again, personality is irrelevant to the truth.

Appeal to Ignorance (argumentum ad ignoratiam)

An appeal to ignorance proposes that we accept the truth of a proposition unless an opponent can prove otherwise. Thus, for example:

• No one has conclusively proven that there is no intelligent life on the moons of Jupiter.
• Therefore, there is intelligent life on the moons of Jupiter.

But, of course, the absence of evidence against a proposition is not enough to secure its truth. What we don’t know could nevertheless be so.

False Cause (post hoc)

The fallacy of false cause infers the presence of a causal connection simply because events appear to occur in correlation or (in the post hoc, ergo propter hoc variety) in temporal succession.

• The moon was full on Thursday evening.
• On Friday morning I overslept.
• Therefore, the full moon caused me to oversleep.

Sometimes evidence adequately supports the conclusion that a causal relationship does exist, but these fallacies clearly are not enough.  They do not fully demonstrate causality.

Begging the Question or Circular Reasoning

Begging the question is the fallacy of using the conclusion of an argument as one of the premises offered in its own support. Although this often happens in an implicit or disguised fashion, an explicit version would look like this:

• All dogs are mammals.
• All mammals have hair.
• Since animals with hair bear live young, dogs bear live young.
• But all animals that bear live young are mammals.
• Therefore, all dogs are mammals.

Unlike the other fallacies we’ve considered, begging the question involves an argument (or chain of arguments) that is formally valid: if its premises (including the first) are true, then the conclusion must be true. The problem is that this valid argument doesn’t really provide support for the truth in its conclusion; we can’t use it unless we have already granted that.

Red Herring

A red herring is a logical fallacy in which irrelevant information is presented alongside relevant information, distracting attention from that relevant information. This may be done intentionally or unintentionally. A red herring is one of the most commonly used fallacies, especially when an arguer has a weak case.

• “I understand you want to know what happened at the embassy. What is really important is to talk about whether the government has enough cash flow to stay open through the month.”
• “Unfortunately, we have to lay off 5% of the workforce. It’s important for us to note that the product we create is exceptionally flawless and we thank our manufacturing department for that.”
• “Why are you pulling me over for speeding? Shouldn’t you be out catching the real criminals? Someone could be robbing a bank as we speak!”

Wishful Thinking (the Is…Ought fallacy)

This fallacy assumes that something Is true because one really thinks it Ought to be true.

• Of course, the soul is immortal. If it were not, then we would all be mere specks of dust in cosmic time.
• Angels do exist. It makes me feel better to think they are with me, so I’m justified in saying they are.
• A personal relationship with God will change your life.