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4.5: Fallacies- Common Problems to Watch For

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    Why assumptions matter

    One of the most powerful techniques for testing whether an argument is valid is to find out what assumptions it makes and check those assumptions.  Noticing and questioning assumptions is a core slow thinking practice in college.  Beyond that, questioning assumptions may be one of the most powerful mental habits a human can learn.  

    In his bestseller The Four Agreements: A Practical Guide to Personal Freedom, Don Miguel Ruiz advises, “Don’t make assumptions.” But we do make assumptions; every belief, every argument depends on them. Perhaps Ruiz’s advice is a shorthand way to ask us to learn to recognize our assumptions so we can decide when to set them aside.

    Questioning assumptions is a habit we may find empowering, freeing, and useful in any area of life. Novelist Isaac Asimov put it another way: “Your assumptions are your windows on the world. Scrub them off every once in a while, or the light won't come in.” Discovering a hidden assumption can be a revelation; it suggests the possibility that things might work another way. There may be other angles to consider.

    Questioning assumptions can be a way to speak back to authority.  Those in power often make assumptions based on their privilege, assumptions that help them stay in power.  They may rationalize their decisions by justifying them with better-sounding reasons.  The white defenders of slavery made a thousand such arguments. Abraham Lincoln responded, “Whenever I hear anyone arguing for slavery, I feel a strong impulse to see it tried on him personally.” Questioning the assumptions of such arguments can help expose their immorality and challenge the power structure.

    Questioning the assumptions of the dominant culture can help anyone less privileged feel personally empowered. Bob Marley famously sang “Emancipate yourselves from mental slavery. // None but ourselves can free our minds.”  

    Questioning assumptions about other people makes it possible to set aside stereotypes and connect authentically.  Michelle Obama, in her book Becoming, calls on us to “fear less, to make fewer wrong assumptions, to let go of the biases and stereotypes that unnecessarily divide us.”

    On a practical level, questioning assumptions can also help us problem-solve. There may be a way around a difficulty if we realize how our assumptions have limited our view.  By first recognizing and then changing our assumptions, we can, to use a common phrase, “think outside the box.”  


    A door slightly open, letting in light. The door has a question mark on it.
    Image by Arek Socha from Pixabay under the Pixabay License.


    What assumptions does the argument make?

    Most arguments don’t mention their assumptions, so summaries don’t typically mention them either.  However, when we assess an argument or write a critical analysis, we need to know whether the argument has a strong foundation.  The reason might seem to prove the claim, but that leap from one to the other depends on assumptions. The writer may not even be aware of these ideas, but they are still necessary to the argument. Philosopher Stephen E. Toulmin popularized the process of finding the assumptions that link the reason to the claim.  He called these warrants, and he found it useful to write them into an argument map such as the one below (see Chapter 2: Reading to Figure out the Argument for more on argument mapping).  


    A claim leads to a reason, supported below by an assumption.
    Text description of argument assumption map

    In the map, the assumption goes underneath with the arrow pointing up because the assumption supports or props up the whole argument.  Note that this assumption is similar to the Golden Rule, “Do unto others as you would have them do unto you.”

    Arguments depend on more than one assumption. The above example also depends on the assumption that "People currently need permission to cross the border legally." We know that under current policy, governments decide whether or not to allow people in. However, it is probably not useful to identify assumptions that are easily verified and not controversial. In fact, we do not need to find every underlying assumption in order to assess an argument; we only need to know which are questionable.

    So if we are reading an argument, how do we identify the assumptions it makes? In a few cases writers will point out their assumptions with phrases like the following:

    • I will assume here _____________.

    • This rests on the assumption that _____________.

    • Of course, this depends on _____________.

    • As we know, _____________.

    • This argument rests upon the idea that _____________.

    • The underlying principle here is that _____________.

    More commonly, though, writers do not state assumptions, sometimes because they seem obvious and sometimes because drawing attention to the assumptions might draw attention to a weakness in the argument. We will need to identify these assumptions on our own. 

    The basic method is to ask ourselves what the reason needs to support the claim. What other idea is necessary for us to make the leap from reason to claim? What underlying idea does that leap depend on? 

    In the border example above, most people would probably agree that we should apply the same standards to ourselves as we do to others. The border argument also depends on the assumption that "People currently need permission to cross the border legally." Both of those assumptions are uncontroversial, so why do we need to talk about them? In fact, we do not need to find every possible underlying assumption in order to assess an argument. We need to focus on assumptions that might not be true or that might not be universally accepted. 

    One way to uncover problematic assumptions is to brainstorm cases where the reason wouldn't necessarily lead to the claim. We can put the reason and the claim into the following question:

    Just because [reason] does that necessarily mean that [claim]?

    If we can think of a case in which the reason doesn't lead to the claim, then there must be a problematic assumption. We can try to find this assumption by filling in the "scenario" blank below with a case in which the reason was true but the claim was not. This technique is sometimes called a scenario test:

    Just because [reason] that doesn't mean [claim] because it could be that...[scenario].

    For example, in the case of the border argument, we could write, "Just because we would feel it was right to cross the border without permission doesn't necessarily mean that we must recognize illegal crossing as ethical because it could be that..." I am tempted to complete this with the idea that "our personal feelings are not always the best guide to ethics." Or I could complete it with a more specific case: "It might be that crossing without permission isn't really right even though some people might feel it is." This helps us to identify another assumption--that we can tell if something is ethical by whether or not it feels right.


    A claim leads to a reason, supported by two assumptions, one with a question mark behind it.
    Text description of the quesitonable assumption map
    A house perched overhanging a cliff.
    Like a house, an argument needs a solid foundation. Photo by Cindy Tang on Unsplash under the Unsplash License.

    Are the assumptions valid?

    Look for exceptions to the assumptions 

    If we want to test an assumption, we can look to cases unrelated to our argument that might prove it wrong. We can probably think of a person who sincerely believes they are doing right, while we are sure their action is unethical. A suicide bomber may believe they are doing God's will by killing people. We can use such a counterexample to help us argue that the assumption in question is not universally true and thus the reason does not necessarily imply the claim.

    Look for evidence for the assumptions

    In the rare case that an argument lists its assumptions and explains why they are justified, we can check whether we find these justifications convincing. More often, though, the author will not have stated the assumptions or provided evidence to support them.  Our critique is a place to call for evidence of any key assumption we have uncovered. We don’t have to make a final pronouncement on whether the assumption is true or not; we may not have formed an opinion on that yet. We may be inclined to doubt it or to believe it, but either way, we should point out when the assumption needs support.  (Philosopher Stephen Toulmin called support for an assumption “backing” so you may see that term used in other rhetoric textbooks).

    For example, the argument below relies on the idea that chronic stress is bad for health:

    Reason:  According to Penn Medicine News, “exposure to weight bias and stigma . . . can lead to a physiological stress response such as increased inflammation and cortisol levels.” 

    Claim: Fat-shaming may be one cause of the health problems associated with being fat. 

    Assumption: Chronic stress leads to health problems.

    We can check if this assumption is valid by consulting a range of reputable medical journals, and we will find many statements and studies that back it up. For example, the popular and well-regarded site includes an article with the title, “Chronic stress puts your health at risk” and lists effects from heart disease to memory impairment.

    Common problems with assumptions 

    Assuming a logical connection where there isn’t one

    Sometimes an argument leaps from a reason to a claim that is not really related. This problem is known generally as a non sequitur, which is Latin for “It does not follow.”  The scenario test will turn up cases like this. Again, we ask ourselves Just because [reason] does that necessarily mean that [claim]? 

    Two posters read, in Spanish and English, "If you are reading this, I am your Brother."
    "Non Sequitur" by mikecogh on Flickr is licensed under CC BY-SA 2.0


    When we use the phrase “non sequitur” in conversation, we usually mean that a statement seems random or out of place. However, in argument non sequiturs often sound convincing and not random at all. On closer examination we see that logically speaking, the reason does not necessarily make the claim true. For example, let’s take the assertion that “Overseas Filipino workers (O.F.W.s) are not citizens in the countries where they work, so they have no legal rights.”  Just because workers are not citizens does not mean that they have no legal rights.  People can have legal rights without being citizens although the law may treat citizens and non-citizens differently in some cases.

    One special case where a logical connection is missing is known as a red herring fallacy or a fallacy of distraction. We commit the red herring fallacy if we attempt to distract the audience from the main thread of an argument, taking things off in a different direction. The diversion is often subtle, with the detour starting on a topic closely related to the original—but gradually wandering off into unrelated territory. The tactic is often, but not always, intentional: if the arguer is not comfortable arguing about a particular topic on the merits, they change the subject to an issue about which they feel more confident and pretend to have won the original argument.

    A red fish intended to be a red herring.
    "Red Herring" by Laurel Russwurm on Flickr is licensed under CC0.

    The red herring fallacy gets its name from the actual fish. When herring are smoked, they turn red and are quite pungent. Stinky things can be used to distract hunting dogs, who of course follow the trail of their quarry by scent; if you pass over that trail with a stinky fish and run off in a different direction, the hound may be distracted and follow the wrong trail. 

    An argument like the following is a good example of the red herring fallacy: 

    Our staff sanitize every classroom surface multiple times throughout the day. Therefore, our school is a leader in stopping the spread of Covid 19.

    During the Covid 19 pandemic, many people and institutions have focused on what Atlantic writer Derek Thompson called “hygiene theater”--the tendency to showcase practices like sanitizing surfaces beyond their actual protective value.  This continued even after scientists had known for some time that airborne transmission was much more common than transmission through surfaces.  The focus on surface hygiene diverted attention away from mask-wearing and ventilation systems, which were more statistically effective in preventing transmission.

    Assuming two things are comparable 

    False analogy

    Many arguments rely on a similarity between two things, usually referred to as an analogy, to conclude that if something is true for one, it will be true for the other. But things that are similar will also have differences, and so for any such argument we need to ask whether there are any differences significant enough to change the outcome. Are the two things really similar enough to justify the conclusion? If not, we have what is often called a false analogy fallacy.

    For example, consider the following argument:

    People have a First Amendment right to express opinions by donating money to political candidates, so corporations should also have that right.

    This is a summary of the decision the Supreme Court came to in the landmark 2010 decision Citizens United vs. FEC. It depends on an analogy between corporations and people, an idea called "corporate personhood" in legal terms. Chief Justice Roberts argued, essentially, that the First Amendment applies to groups of people, such as corporations, as well as to individuals. Now, is this a fallacy? In a dissenting opinion, Justice Stevens argued that the Constitution was intended to apply to "We, the People," not corporate entities. He listed the key differences that led him to conclude that:

    In the context of election to public office, the distinction between corporate and human speakers is significant. Although they make enormous contributions to our society, corporations are not actually members of it. They cannot vote or run for office. Because they may be managed and controlled by nonresidents, their interests may conflict in fundamental respects with the interests of eligible voters. The financial resources, legal structure, and instrumental orientation of corporations raise legitimate concerns about their role in the electoral process. Our lawmakers have a compelling constitutional basis, if not also a democratic duty, to take measures designed to guard against the potentially deleterious effects of corporate spending in local and national races.

    As we can see, Justice Stevens thought that this was a case of false analogy, but Chief Justice Roberts disagreed. Introducing the fallacy label here does not resolve the debate, but it may help to clarify where the disagreement lies.

    An apple next to an orange.
    Are apples and oranges too different to compare? "Like apples and oranges" by Sarah Braun on Flickr is licensed under CC BY NC SA 2.0.


    Slippery slope

    One type of invalid comparison comes in arguments that make a dramatic prediction that if one thing happens, other more dramatic things will inevitably follow.  It depends on the idea that the first event is comparable to the other, more dramatic events. A slippery slope argument claims a disastrous cascade effect will take place if we take certain action. It presents a chain of events leading to disaster as if it is unstoppable or highly probable. But how slippery is the slope really? How likely is the disaster? Are there factors that could stop the chain reaction?

    For example, take the following argument:

    If we allow people to self-identify their gender regardless of their biology, they will expect to be able to self-identify their race and then their age and species. Next thing we know, the law will demand we pretend a person is a gorilla!

    There is a certain appealing momentum to these arguments: we imagine a boulder rolling down a hill.  But will one thing really lead to another? Just because we can imagine that one thing might lead to another does not mean that it inevitably will. Many transgender people already legally claim a gender identity different from the one they were assigned at birth. However, very few people believe themselves to be a different age than their chronological age or a species other than human.  There is no movement to push for legal recognition of self-identified age or species.

    Boulder poised to fall on a mostly flat, rocky area.
    If it falls, this boulder will not roll far; the slope is not slippery. Photo by Prashant Kumar on Pixahive, licensed under CC0.

    Fallacious slippery slope arguments have long been deployed to resist social change. Those opposed to the abolition of slavery warned of economic collapse and social chaos. Those who opposed women’s suffrage asserted that it would lead to the dissolution of the family, rampant sexual promiscuity, and social anarchy. Of course, none of these dire predictions came true; the slopes weren’t slippery.

    We can critique a slippery slope argument with phrases like the following:

    • The argument claims that _____________ will inevitably lead to _____________, but this is far from certain.  

    • They assume that _____________ will set off a chain reaction leading to _____________; however this is unlikely because _____________.

    (We can also think of the slippery slope arguments as committing the fallacy of false analogy. We saw above that one argument against letting people identify their own gender depended on a comparison between gender identity and other forms of identity such as species and age, but most would argue that key differences between these categories would lead to different legal treatment. The differences would stop the slide down the slope. Faulty arguments can often legitimately be classed under different fallacy labels.)

    Assuming that one thing causes another 

    Arguments often claim casually that an earlier event caused a later event. To be sound, such arguments need quite a bit of support. They need to show that there is a likely way in which the first event could cause the second. They need to ask if something else have caused the second event. Could a third factor have caused both events? Maybe the first event contributed to the second, but other factors did as well. Or maybe there is no link between the two events at all.

    Assuming that a first event caused a second without further justification is a fallacy variously referred to as false cause, doubtful cause, post hoc ergo propter hoc, post hoc, post hoc reasoning, or with the catchphrase “correlation is not causation.” Once we look for it, we see it everywhere, including on the news and in reputable academic settings. 

    One example lies in the way we evaluate the performance of presidents of the United States. Everything that happens during or immediately after their administrations tends to be pinned on them. But presidents aren’t all-powerful; they don’t cause everything that happens during their presidencies. Similar claims on behalf of state governors are even more absurd. At the 2016 Republican National Convention, Governors Scott Walker and Mike Pence—of Wisconsin and Indiana, respectively—both pointed to record-high employment in their states as vindication of their conservative policies. But some other states were also experiencing record-high employment at the time: California, Minnesota, New Hampshire, New York, Washington. Yes, they were all controlled by Democrats. Maybe there’s a separate cause for those strong jobs numbers in differently governed states? Possibly it has something to do with the improving economy and health of the job market in the country as a whole.

    Proving that one thing caused another can be tricky. We talk more about various strategies for showing causation in Section 7.5: Causal Arguments.

    Dialogue showing an example of the correlation is not causation fallacy.
    The listener here hasn’t grasped the concept that just because the class came first doesn’t mean the understanding came from the class. Image by Liz T on Flickr, licensed under CC BY 2.0. Full description of the correlation is not causation image.

    Phrases for critiquing assumptions

    Once we have identified an assumption that we want to question, we can introduce the assumption and explain its weakness with it with phrases like the following:

    • _____________relies on the idea that _____________; however, _____________.

    • The argument assumes that _____________ without providing evidence.

    • _____________takes for granted that _____________, but we may wonder whether this is a justified assumption because_____________.

    • _____________depends on the assumption that_____________. Is this always the case? Some might say that _____________.

    • _____________ depends on a belief in _____________, which may not be shared by all readers because _____________.

    • The underlying idea here is that _____________; however we must ask ourselves whether _____________.

    • The implicit assumption is that _____________ but some may question whether, in fact, _____________.


    The above is original content by Anna Mills, except for the descriptions of the red herring, slippery slope, and post hoc ergo propter hoc fallacies, which Anna Mills adapted from the "Informal Logical Fallacies" chapter of Fundamental Methods of Logic by Matthew Knachel, UWM Digital Commons, licensed CC BY.