Chapter 14: Logical Reasoning

14.1 – What is Correct Reasoning?

We have seen that logos involves composing a speech that is structured in a logical and easy-to-follow way; it also involves using correct logical reasoning and, consequently, avoiding fallacious reasoning or logical fallacies.

Analogical reasoning is one of several types of logical reasoning methods that can serve us well if used correctly, but it can be confusing and even unethical if used incorrectly. In this chapter we will look first at “good” reasoning and then at several of the standard mistakes in reasoning, which are called logical fallacies. In higher education, teaching and learning critical thinking skills are a priority and those skills are one of the characteristics that employers are looking for in applicants (Adams, 2014).

Involved in critical thinking are problem-solving and decision-making, the ability to evaluate and critique based on theory and the “knowledge base” (what is known in a particular field), skill in self-reflection, recognition of personal and societal biases, and the ability to use logic and avoid logical fallacies. On the website Critical Thinking Community, in an article entitled “Our Concept and Definition of Critical Thinking” (2013), the term is defined this way:

Critical thinking is that mode of thinking — about any subject, content, or problem — in which the thinker improves the quality of his or her thinking by skillfully analyzing, assessing, and reconstructing it. Critical thinking is self-directed, self-disciplined, self-monitored, and self-corrective thinking. It presupposes assent to rigorous standards of excellence and mindful command of their use. It entails effective communication and problem-solving abilities, as well as a commitment to overcome our native egocentrism and sociocentrism.

Critical thinking is a term with a wide range of meaning, one of which is the traditional ability to use formal logic. To do so, you must first understand the two types of reasoning: inductive and deductive.

14.2 – Inductive Reasoning

Inductive reasoning (also called “induction”) is probably the form of reasoning we use on a more regular basis. Induction is sometimes referred to as “reasoning from example or specific instance,” and indeed, that is a good description. It could also be referred to as “bottom-up” thinking. Inductive reasoning is sometimes called “the scientific method,” although you don’t have to be a scientist to use it, and use of the word “scientific” gives the impression it is always right and always precise, which it is not.

Inductive reasoning happens when we look around at various happenings, objects, or behaviour, and see patterns. From those patterns we develop conclusions. There are four types of inductive reasoning, based on different kinds of evidence and logical moves or jumps.


Generalization is a form of inductive reasoning that draws conclusions based on recurring patterns or repeated observations. To generalize, one must observe multiple instances and find common qualities or behaviours and then make a broad or universal statement about them. If every dog I see chases squirrels, then I would probably generalize that all dogs chase squirrels.

If you go to a certain business and get bad service once, you may not like it. If you go back and get bad treatment again, you probably won’t go back again because you have concluded “Business X always treats its customers poorly.” However, according to the laws of logic, you cannot really say that; you can only say, “In my experience, Business X treats its customers poorly” or more precisely, “has treated me poorly.” Additionally, the word “poorly” is imprecise, so to be a valid conclusion to the generalization, poorly should be replaced with “rudely,” “dishonestly,” or “dismissively.” The two problems with generalization are over-generalizing (making too big an inductive leap, or jump, from the evidence to the conclusion) and generalizing without enough examples (hasty generalization, also seen in stereotyping).

In the example of the service at Business X, two examples are really not enough to conclude that “Business X treats customers rudely.” The conclusion does not pass the logic test for generalization, but pure logic may not influence whether or not you patronize the business again. Logic and personal choice overlap sometimes and separate sometimes. If the business is a restaurant, it could be that there is one particularly rude server at the restaurant and he happened to wait on you during both of your experiences. Everyone else may get fantastic service, but your generalization was based on too small a sample.

Causal reasoning

Instead of looking for patterns the way generalization does, causal reasoning seeks to make cause-effect connections. Causal reasoning is a form of inductive reasoning we use all the time without even thinking about it. If the street is wet in the morning, you know that it rained based on past experience. Of course, there could be another cause—the city decided to wash the streets early that morning—but your first conclusion would be rain. Because causes and effects can be so multiple and complicated, two tests are used to judge whether the causal reasoning is valid.

Good inductive causal reasoning meets the tests of directness and strength. The alleged cause must have a direct relationship on the effect and the cause must be strong enough to make the effect. If a student fails a test in a class that he studied for, he would need to examine the causes of the failure. He could look back over the experience and suggest the following reasons for the failure:

  1. He waited too long to study.
  2. He had incomplete notes.
  3. He didn’t read the textbook fully.
  4. He wore a red hoodie when he took the test.
  5. He ate at a dodgy restaurant the night before.
  6. He only slept four hours the night before.
  7. The instructor did not do a good job teaching the material.
  8. He sat in a different seat to take the test.
  9. His favourite football team lost its game on the weekend before.

Which of these causes are direct enough and strong enough to affect his performance on the test? All of them might have had a slight effect on his emotional, physical, or mental state, but all are not strong enough to affect his knowledge of the material if he had studied sufficiently and had good notes to work from. Not having enough sleep could also affect his attention and processes more directly than, say, the a hoodie or a football game. We often consider “causes” such as the colour of the hoodie to be superstitions.

Taking a test while sitting in a different seat from the one where you sit in class has actually been researched (Sauffley, Otaka, & Bavaresco, 1985), as has whether sitting in the front or back affects learning (Benedict & Hoag, 2004). (In both cases, the evidence so far says that they do not have an impact, but more research will probably be done.) From the list above, #1-3, #6, and #7 probably have the most direct effect on the test failure. At this point our student would need to face the psychological concepts of locus of control and responsibility—was the failure on the test mostly his doing or his instructor’s?

Causal reasoning is susceptible to four fallacies: historical fallacy, slippery slope, false cause, and confusing correlation and causation. The first three will be discussed later, but the last is very common, and if you take a psychology or sociology course, you will study correlation and causation, as well. This video of a Ted Talk ( will explain the concept in an entertaining manner. Confusing correlation and causation is the same as confusing causal reasoning and sign reasoning, discussed below.

Sign Reasoning

Right now, as the author is writing this chapter, the leaves on the trees are turning brown, the grass does not need to be cut every week, and geese are flying south. These are all signs of fall in this region. These signs do not make fall happen and they don’t make the other signs—cooler temperatures, for example—happen. All the signs of fall are caused by one phenomenon: the revolution of the earth around the sun and earth’s tilt on its axis, which make shorter days, less sunshine, cooler temperatures, and less chlorophyll in the leaves, leading to red and brown colours.

Confusing signs and causes is easy. Sign reasoning, then, is a form of inductive reasoning in which conclusions are drawn about phenomena based on events that precede or co-exist with, but not cause, a subsequent event. Signs are like the correlation mentioned above under causal reasoning. If someone argues, “In the summer more people eat ice cream, and in the summer there is statistically more crime. Therefore, eating more ice cream causes more crime” (or “more crime makes people eat more ice cream”), that, of course, would be silly. These are co-occurrences—signs—but they are effects of something else: hot weather. If we see one sign, we will see the other. Either way, they are signs of perhaps two different events that just happen to be occurring at the same time, but not causes of each other.

Analogical reasoning

As mentioned above, analogical reasoning involves comparison. For it to be valid, the two subjects (schools, states, countries, businesses) must be truly alike in many important ways. Although Harvard University and your college are both institutions of higher education, they are not essentially alike in very many ways. They may have different missions, histories, governance, surrounding locations, sizes, clientele, stakeholders, funding sources, and funding amounts. So arguing that “Harvard has a law school; therefore, since we are both educational institutions, my college should have a law school, too” would be a premature conclusion at best. On the other hand, there are colleges that are very similar to your college in all those ways, so comparisons could be valid in those cases.

You have probably heard the phrase, “that is like comparing apples and oranges.” When you think about it, though, apples and oranges are more alike than they are different (they are both still fruit, after all). This observation points out the difficulty of analogical reasoning—how similar do the two comparators need to be for a valid analogy? Second, what is the purpose of the analogy? Is it to prove that one college has a specific program (sports, a math club, a theatre major), therefore, another college should have that program, too? Are there other factors to consider? Analogical reasoning is one of the less reliable forms of logic, although it is used frequently.

14.3 – Deductive Reasoning

The second type of reasoning is called deductive reasoning, or deduction, a type of reasoning in which a conclusion is based on the combination of multiple premises that are generally assumed to be true. It has been referred to as “reasoning from principle,” which is a good description. It can also be called “top-down” reasoning. However, you should not think of deductive reasoning as the opposite of inductive reasoning. They are two different ways of thinking about evidence.

First, formal deductive reasoning employs the syllogism, which is a three-sentence argument composed of a major premise (a generalization or principle that is accepted as true), a minor premise (an example of the major premise), and a conclusion. This conclusion has to be true if the major and minor premise are true; it logically follows from the first two statements. Here are some examples. The most common one you may have seen before.

All men are mortal. (Major premise: something everyone already agrees on)

Socrates is a man. (Minor premise: an example taken from the major premise.)

Socrates is mortal. (Conclusion: the only conclusion that can be drawn from the first two sentences.)

Major Premise: All Douglas College students must take DCUE 0100.

Minor Premise: Amir is a Douglas College student.

Conclusion: Amir must take DCUE 0100.

Of course, at this point you may have some issues with these examples. First, Socrates is already dead and you did not need a syllogism to know that. The Greek philosopher lived 2,400 years ago! Second, these seem kind of obvious. Third, are there some exceptions to “All Douglas College students must take DCUE 0100”? Yes, if they started their studies here far enough in the past, they would not have completed DCUE 0100. So while all men are mortal, not all Douglas College students must complete DCUE 0100.

Consequently, the first criterion for syllogisms and deductive reasoning is that the premises have to be true for the conclusion to be true, even if the method is right. A right method and untrue premises will not result in a true conclusion. Equally, true premises with a wrong method will also not result in true conclusions. For example:

Major premise: All dogs bark.

Minor premise: Fifi barks.

Conclusion: Fifi is a dog.

You should notice that the minor premise is stated incorrectly. We know other animals bark, notably seals (although it is hard to think of a seal named “Fifi”). The minor premise would have to read “Fifi is a dog” to arrive at the logical conclusion, “Fifi barks.”

14.4 – Logical Fallacies

The second part of achieving a logical speech is to avoid logical fallacies. Logical fallacies are mistakes in reasoning. While reviewing dozens of logical fallacies here is tempting, an even better option is to direct you to this excellent website:

This website offers explanations and examples of 24 of the most common logical fallacies. Take some time to study and understand each one.

That website, while fantastic is missing several other important logical fallacies, which are explained below.

False Analogy

A false analogy is a fallacy where two points are compared that do not share enough similarities to be compared fairly. As mentioned before, for analogical reasoning to be valid, the two points being compared must be essentially similar—similar in all the important ways.

Post hoc ergo propter hoc

This long Latin phrase means “After the fact, therefore because of the fact.” In this fallacy, one event happens and then a second event happens; the fallacy states that, because one event happened first, it is the cause of the second event. As an example, if you sign up for a public speaking course and then the instructor is sick on the first day of classes, you signing up for the course did not cause the instructor to be sick (no matter how many times it may coincidentally happen to you).

Argument from Silence

You can’t prove something from nothing. If you have no evidence on a matter, then that is all you know. You cannot conclude anything based on a lack of evidence (except that you have no evidence). We also need to differentiate between something being fallacious and something being false. A fallacious argument has incorrect reasoning, regardless of the truth of the facts asserted or the conclusion reached. If I point to a young woman on campus and say, “That’s Taylor Swift,” I am simply stating a falsehood, not committing a fallacy. If I say, “Her name is Taylor Swift and the reason I know that is because no one has ever told me that her name is not Taylor Swift” (argument from silence), that is a fallacy and a falsehood (unless by some odd circumstance her name really is Taylor Swift or the singer Taylor Swift frequents your campus).

Non Sequitur

Non sequitur is Latin for “it does not follow.” It’s an all-purpose fallacy for situations where the conclusion sounds good at first, but then you realize there is no connection between the premises and the conclusion. If you say to your supervisor, “I need a raise because the price of BMWs went up,” that is a non sequitur.

Inappropriate Appeal to Authority

There are appropriate appeals to authority, such as when you use sources in your speech who are knowledgeable, experienced, and credible. But not all sources are credible. Some may be knowledgeable about one field, but not another. A person with a Nobel Prize in economics is not qualified to talk about medicine, no matter how smart they are (the economist could talk about the economics of the healthcare sector, however). Of course, the most common place we see this is in celebrity endorsements on commercials.

Appeal to Tradition

Essentially, appeal to tradition is the argument, “We’ve always done it this way.” This fallacy happens when traditional practice is the only reason for continuing a policy. We have many traditional practices that are wonderful and that makes us feel good, but doing something only because it’s always been done a certain way is not a strong argument. Does it work? Is it cost effective? Is some other approach better? If your college library refused to adopt a computer database of books in favour of the old card catalogue because “that’s what libraries have done for decades,” you would likely argue they need to get with the times. The same would be true if the classrooms all still had only chalkboards instead of computers and projectors and the administration argued that it fit the tradition of colleges.

Red Herring

This one has an interesting history and you might want to look it up. A herring is a fish and it was once used to throw off or distract foxhounds from a particular scent. A red herring, then, is creating a diversion or introducing an irrelevant point to distract someone or get someone off the subject of the argument. When a politician in a debate is asked about her stance on immigration and the candidate responds, “I think we need to focus on reducing the debt,” she is introducing a red herring to distract from the original topic under discussion. If someone argues, “We should not worry about the needs of people in other countries because we have poor people in the Canada,” that may sound good on the surface, but it is a red herring and a false dilemma (either-or) fallacy. Addressing poverty in this country and other countries is possible at the same time.

Guilt by Association

This fallacy is a form of false analogy based on the idea that if two points bear any relationship at all, they are comparable. No one wants to be blamed for something just because they were in the wrong place at the wrong time or happen to bear some resemblance to a guilty person. An example would be if someone argued, “Adolf Hitler was a vegetarian; therefore being a vegetarian is evil.” Of course, vegetarianism as a life practice had nothing to do with Hitler’s character. Although this is an extreme example, it is not uncommon to hear guilt by association used as a type of ad hominem argument. There is actually a fallacy called “reductio ad Hitlerum”—whenever someone dismisses an argument by bringing up Hitler out of nowhere. (Tip: never compare anybody or anything to Hitler or the Nazis. It never works for you.)


This chapter took the subject of public speaking to a different level in that it was somewhat more abstract than the other chapters. However, a public speaker is responsible for using good reasoning as much as she is responsible to have an organized speech, to analyze the audience, or to practice for effective delivery.

Something to Think About

You cannot hear logical fallacies unless you listen carefully and critically. Keep your ears open to possible uses of fallacies. Are they used in discussion of emotional topics? Are they used to get compliance (such as to buy a product) without allowing the consumer to think about the issues? What else do you notice about them?


This chapter was adapted from Exploring Public Speaking, 4th Edition by Barbara Tucker and Matthew LeHew, which is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.


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Public Speaking for Today's Audiences Copyright © 2023 by Sam Schechter is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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