Argument by Analogy: This is a very common form of argument, especially in moral and political issues. The argument begins with a case, sometimes hypothetical, which everyone agrees has some quality. Next, the proponent says that this case is like the case at hand about which he is trying to convince you. Finally, the proponent argues that because these cases are similar, the quality from the first case must carry over to the second. Utilitarian philosopher Peter Singer says suppose you are on your way to a lecture and see a child drowning in a small pond, but stopping to save the child would ruin your new shoes and make you late. He suggests that not saving the child under those conditions would be monstrous. But then, he argues so is going about our busy lives and chasing trivial pursuits while children in the third world die of preventable diseases and malnutrition. Just as we are obligated to help the fictitious drowning child, we are obligated to help the "drowning" children in impoverished countries.

Reductio Ad Absurdum: In this form of argument, you start with your opponent's view, assume for the sake of argument that it's true, then show it implies something totally implausible that no one could possibly believe. For example, suppose someone's a strong Second Amendment advocate and says that there should be no restrictions on firearms because it's the last line of defense against a tyrannical government. But if you believe that, you believe that people should be able to have weapons of the type that could defend against a military takeover. That in turn implies that people should be able to have fully automatic machine guns, bazookas and anti-aircraft guns. But it's ridiculous that someone be able to walk into a store and purchase military armaments, capable of inflicting casualties on a massive scale, so there must be some restrictions on firearms, whatever the Founding Fathers' motivations for the Second Amendment. You might see this as an opposite approach to argument by analogy. In argument by analogy, you start with a case that everyone agrees with and try to show that your argument is just like it; in reductio ad absurdum, you take your opponent's case and show that it is like a case no one agrees with.


Mistakes in reasoning can be classified into two kinds. Formal fallacies show a mistake which can be put in symbolic form and are mistakes in deductive logic. Deductive arguments are either valid or invalid, depending on whether their premises guarantee their conclusions. We looked at two formal fallacies, denying the antecedent and assorting the consequent, which masquerade as the valid argument forms modus ponens and modus tollens. Informal fallacies are mistakes in inductive reasoning and also often mimic logically sound forms of reasoning.

Appeal to authority: Appeals to experts are appropriate, especially when giving supportive evidence in an argument. However, it is tempting to forgo looking at the evidence oneself and to defer to experts. Accepting the conclusion of an authority without looking at any of the particulars of the evidence is usually problematic. When considering the use of an authoritative source of evidence in an argument, ask yourself the following five questions: 1. Are there genuine experts in this field? 2. Is this person or organization recognized by others in the field as having expertise? 3. Are they an expert on this particular subject? 4. Is the argument relying on the say-so of an expert when the evidence is widely available and easily understandable to a layman? If the answer to any of these questions is no, then appealing to such a person or organization commits this fallacy.

For example, suppose someone says, "The Pastor at my Church says the Bible teaches that homosexuality is a sin." You ask where exactly in the Bible homosexuality is condemned, and your friend says, "Oh, I don't know. It's in there some place. I trust Pastor Bob; he has a degree from Dallas Theological Seminary." This would be a fallacious appeal to authority because your friend or any layperson could look up the mentions of homosexuality in the Bible in the Old and New Testaments in an online concordance, Bible dictionary or encyclopedia and read them for themselves.

Is the expert in an ideologically charged field in which there may be institutional bias or do they have an obvious vested interest, such that their expert opinion may not be reliable? Sometimes this is difficult to tell. Institutions are often founded and funded in order to promulgate certain beliefs. Funding has to come from somewhere. Even if the original aims are objective, a research institution may develop points of view over time due to people hiring like-minded researchers or where donations come from.  For example, the Tobacco Institute used to fund and release studies which coincidentally, of course, showed that smoking or second hand smoke wasn't that bad for you. Thinking critically means looking at such studies with a jaundiced eye. Similarly, the meat, egg and dairy industry funds studies that show that consuming their product is not going to give you diabetes, heart disease or cancer. One of the reasons to be suspicious of these studies is that there are large bodies of studies which contradict their findings. This isn't the case with nuts, which the evidence shows are generally good for you. If the nut industry funds studies on the health benefits of eating nuts, should you automatically dismiss them? Maybe nuts are good for you and the studies are done in good faith and provide evidence for this. The devil is often in the details of the study. Being cautious about industry funded studies is wise, but if you completely dismiss a study based on its funding source without actually looking at the evidence, you are guilty of the ad hominem circumstantial fallacy below.

Another common form of appeal to authority refers to the identity of the individual, especially as a victim or oppressed class. You will often hear, "As a [oppressed class], I feel that [belief]." Often they claim to have special moral authority to speak on the issue. However, if you think about it logically, the messenger of an argument is completely irrelevant to whether the premises are true or the reasoning is good. If you reject my argument because of who I am, suppose I just write out my argument and hand it to someone you sdem appropriate to deliver it. Do my premises suddenly go from being false to true, my reasoning from fallacious to sound? What if an argument is written anonymously. Would you have to know who wrote it in order to know whether it was good or bad? Of course not. Being black, an immigrant, a woman, gay or disabled doesn't accord you expert opinion status or give you any special authority when it comes to speaking to contentious social issues. A person's identity may have rhetorical value but it has no evidential or logical value. An argument stands or falls on its merits, based on the quality of the evidence for its premises and the strength of its logical reasoning.

Ad hominem: Literally "against the man." This is when you attack the character or motives of the person making the argument rather than the argument itself. The ad hominem abusive form is when you tar your opponent as a nazi, racist, sexist, homophobe, xenophobe, anti-immigrant, anti-woman, fat, ugly, stupid, etc. instead of addressing his argument. Note that it's only an ad hominem fallacy if you are attacking someone's irrelevant personal characteristics as a substitute for addressing the substance of their argument. Ad hominem circumstantial is when you insinuate that the person has some nefarious or self-serving motive for believing something. It can also take the form of requiring some sort of experiential or moral standing in order have a view on a subject. So for example, someone might object to a man expressing a prolife position "because he can't get pregnant" or a person with a view on the military who has "never served." An argument is sound or unsound based on its contents, not on who utters it (as was seen in fallacious appeals to authority mentioned above). To see this is so, consider that any person making an argument could simply have an ally of the preferred identity or circumstances utter exactly the same words. Do the words mean something different because they came out of someone else's mouth? Do the ideas they have expressed change? Do their premises or reasoning become any better or worse? Of course not. Chances are, if you think identity matters in logical arguments, you're trying to think with your emotions or allowing yourself to be manipulated by rhetoric.

Guilt by Association: This fallacy is similar to the ad hominem attack, but instead of attacking the character of the person making or representing an argument, you attack someone associated with that person or attempt to connect that person with some person or group with a bad reputation. Suppose someone makes an argument for new and stricter gun laws, citing the relative ease with which criminals or the mentally ill can obtain weapons. Suppose you counter that that Hitler was in favor of gun control! If you did, you'd be committing the fallacy of guilt by association. Just because Hitler favored gun control doesn't necessarily mean it's bad. Hitler was a vegetarian and a dog lover, too, but that doesn't imply that vegetarianism or having a dog is bad. You brought up Hitler's support for gun control to tarnish the idea with his bad reputation instead of addressing the argument. Instead of bringing up Hitler, you could have argued that exiting laws already bar criminals and the mentally ill from having weapons and that what we need is better enforcement of existing laws, not new laws restricting the rights of law-abiding citizens.

A classic example of this fallacy is the last Presidential election, in which CNN ran story after story about marginal Louisiana politician and former KKK Grand Wizard David Duke and small, fringe white identity groups with little support or influence supporting candidate Donald Trump. Liberal groups like the Southern Poverty Law Center and various Democrat-friendly commentators attempted to insinuate that Trump was running on a platform of white supremacy, despite Trump's repeated statements that he didn't have anything to do with them and repeatedly disavowing them. If, instead of looking at his record of involvement in civil rights issues before becoming a candidate or his actual platform or speeches, you decided candidate Trump was not worthy of your vote based on support from David Duke, you would have committed the fallacy of guilt by association. Along the same lines, the fact that serial abuser of women Harvey Weinstein was a big Democratic donor and supporter of Hillary Clinton is irrelevant to any of the arguments Hillary made about women's issues on the campaign trail or whether she is worthy of your vote, especially if it hasn't been shown she knew of the abuse.

Jesus was criticized for eating with sinners and famously answered, "It's not those who are healthy who need a physician, but those who are sick." In our increasingly polarized world, we shouldn't be afraid to engage those we disagree with in civil dialogue. Maybe we'll convince them. Maybe we'll learn something. Maybe they will see the light and reform. On the other hand, if someone, closely associates with, supports and fails to criticize someone guilty of reprehensible acts or views or is a member of a group with an odious purpose or history, that person may be legitimately criticized on that basis.

Tu quoque: Literally "You, too." This argument attempts to distract from a problematic argument by pointing out that the other side "does it, too." For example, suppose someone brings up the topic of female "circumcision" in the Middle East as a violation of human rights. A defender of the societies who practice it might bring up male circumcision as practiced in the United States. Whether male circumcision is bad has no bearing on whether female "circumcision" is bad.

Lack of proportion: This is where two cases are compared which are very different in degree. In the above example, the procedure lops off the clitoris and removes or sews up the vaginal lips. An analogous case in men would be castration or cutting off the head of the penis. This is why this procedure is more commonly referred to as FGM or female genital mutilation, not circumcision. Circumcision is an unnecessary surgery on a child, done without consent, and does take away some sensation, but is not comparable to FGM in the extent of pain or damage to normal function.

Straw man: This is when you mischaracterize an opponent's view in order to more easily attack it. This is an old medieval combat metaphor. When practicing with weapons, instead of fighting a real opponent, you'd attack a suit of clothes stuffed with straw, which would represent him. Those who commit this fallacy metaphorically impale a straw man who can't fight back and declare victory, while ignoring their real opponent. It's much easier to attack an absurd, exaggerated version of your opponent's argument than their real argument.

Red herring: This is when someone brings up a seemingly related but irrelevant topic in order to distract from the argument at hand. The name comes from a technique used to train hunting dogs. The dogs would be following some scent and the trainer would drag a ripe, pungent fish across the trail in order to to draw the dogs off the trail. Similarly, an opponent may try to take you "into the weeds" with some other issue which has no bearing on the subject. For example, during an abortion protest, someone might say "Why don't you help the children who are already born?" Whether children who are already born are receiving sufficient care is irrelevant to the topic of whether unborn children deserve consideration or have a right to life.

Ad Populum/Bandwagon: Everyone does or believes something, so you should, too. For example, someone might argue that every modern, industrialized country has socialized medicine, so the U.S. should, too. Or she might argue that no other modern, industrialized country has the prison population that the U.S. has, therefore we should have fewer prisoners. Sometimes this is used as an excuse for bad behavior. "There's nothing wrong with taking a few office supplies from work; everybody does it."

Slippery slope: This is the argument that a first step will lead to a cascading series of steps culminating in some catastrophe. For example, a person might argue that gun registration will lead to gun confiscation and then a police state like North Korea. Sometimes small steps do lead to cascading negative effects. Taxes and and the creation of government agencies are like this, as are new laws and regulation. But in the foregoing example, a lot has to be assumed to fill in the steps. For example, we've had gun registration for some time but there has been no mass confiscation, and parts of Europe, which have largely confiscated private firearms, are not police states like North Korea.