The post on the DNA of Argument introduced the role reasoning played in argument. I wrote that reasoning connected the evidence someone presents to the assertion someone makes. You could also think of reasoning as a bridge or a link. In this post, we take a closer look at some different types of reasoning and how to be on your guard against obtuseness and deceit. We’ll look at both deductive (moving from the general to the specific) and inductive (moving from the specific to the general) reasoning.

Deductive reasoning

Deductive reasoning uses a major premise (rule, regulation, law or definition) the audience generally accepts or recognizes to show that an instance or example falls within the parameters of that major premise. If you accept the major and minor premise, then logic demands you accept the conclusion as well. Let’s look at a couple of examples. In outline form, the first would look like this:

Major Premise: Disclosing classified information is against the law

Minor Premise: Hilary Clinton disclosed classified information in her emails

Conclusion: Hilary Clinton broke the law

Few people would argue against the major premise, so the only option to confront the logical conclusion would be to argue against the minor premise, i.e. Clinton did not disclose classified information in her emails. (As an aside, whether the minor premise is true is an unsettled question. Although the Benghazi hearings failed to uncover any wrongdoing, Clinton remains the subject of an F.B.I. investigation). The second example in outline form would be:

Major Premise: All Democrats are liberals

Minor Premise: Jim Webb is a Democrat

Conclusion: Jim Webb must be a liberal

For purposes of illustration, let’s take the term “liberal” to mean someone who is dovish on use of the military, progressive on social issues and optimistic about the role of government. Since we know the minor premise is true, avoiding the logical conclusion would entail arguing against the major premise (the definition of liberal). Anyone who watched the 1st Democratic debate would be hard pressed to label Webb a liberal.

What to Watch for in Deductive Reasoning

Would a reasonable audience generally accept the major premise? If it’s a rule, law or regulation, there’s little room for debate. If it’s a definition or value, you must work to make sure it enjoys audience acceptance. One of the ways to do this is to show the audience the value you’re asking them to accept is consistent with a value they already hold, or that there is another value which should take priority. When the audience accepts the major premise, but disagrees with the conclusion, the minor premise becomes the axis on which the argument turns. Look at the evidence you present, make sure it’s free from fallacies and can be checked against the facts.

Inductive Reasoning

Inductive generalizations take the form of reasoning by sign, analogy or cause.

Reasoning from Signs. Think of the last time you visited the doctor’s office. Unless you were there for a general checkup, someone (doctor, PA, nurse) probably asked “What brings you in here today?” In other words, you’re being asked what symptoms (signs) are giving you cause for concern. Upon further questioning, the doctor will arrive at a probable diagnosis. If you say that you’ve been experiencing vomiting, diarrhea, chills, congestion and a fever, the doctor might take these as signs of the flu. You’ll find the same type of question (What are you concerned about?) being asked by a competent mechanic when you take your vehicle in to be repaired or by someone behind the genius bar at the Apple Store.

During the primary debates, candidates pointed to pushing the Iran nuclear deal [Webb, 1st Democratic debate] a smaller military [Carson, 1st Republican debate] an unstable foreign policy [Rubio, 3rd Democratic debate] and a failure to confront Putin [Fiorina, 4th Republican debate] as signs that America’s standing has decreased in the world under the leadership of President Obama.

What to Watch for in Reasoning by Signs

Signs can be either certain or probable. If you’ve ever watched Criminal Minds or some variation of CSI, you know that it’s almost over when they have a fingerprint or DNA sample from an unsub because these are certain signs of someone’s identity. Probable signs (such as those in the example above) may need further argument to convince the audience to accept the major premise. For example someone could argue that the signs cited from the debates show not America’s decreased standing, but instead, America’s use of diplomacy.

Reasoning from Analogy. In this type of reasoning, someone argues that two things are similar enough that we can reasonably compare them and make inferences about one from the other. Have you ever had to stand in a long line at the Post Office while only one or two employees were waiting on people, even though you saw several more employees in the back? If you expect the same experience at any Post Office you visit, you’re reasoning by analogy.

Trump reasoned by analogy when he used the example of Eisenhower’s “Operation Wetback” program to deport foreign workers. [Trump uses Eisenhower analogy ] Several candidates have also used the analogy of sending ground troops to Syria as the beginning of another quagmire similar to the Iraq war.

What to Watch for in Reasoning by Analogy

The two biggest questions to ask are 1) Are the examples used similar in significant ways? 2) Are the examples used dissimilar in only insignificant ways? Trump has been challenged on both the numbers he used and the humaneness of the process in Operation Wetback. [ Read the full Politifact story here ]

Reasoning from Cause. Used when one is making a causal assertion, this type of reasoning argues that A causes B, or B is the effect of A. Having done my fair share of traveling, I have often suffered the misfortune of food poisoning. On these occasions, I try to whittle down the possible causes to arrive at a conclusion. “The person traveling with me is not sick. What did I eat that person didn’t? Was there something different about the cutlery we used or the dishes we ate from? How long after eating different things did I start to feel the effects?”

All of the Republican candidates have asserted that Obama’s economic and social agenda (increased regulation, Obamacare, more spending, etc. have caused (declining wages, fewer jobs, a stagnant economy, larger deficits, etc.). On the Democratic side, they too recognize some problems with the economy, but their most frequently invoked causal factors are the greed, recklessness and unfair advantages of Wall Street, along with the wealthy paying too few taxes. (To be fair, Trump has also said the wealthy pay too few taxes).

What to Watch for in Reasoning by Cause.

Simply because one thing follows another in time doesn’t mean the first caused the second (This is a post hoc fallacy whose full name post hoc ergo propter hoc literally means “after this, therefore because of this”). Also ask yourself if there is any other factor or factors that could contribute to an effect. As Democrats are quick to point out, Obama inherited an economic recession from Bush ’43—just as the 45th president will most likely inherit a Middle Eastern morass from Obama. Also ask yourself if the things are simply appearing at the same time (correlation rather than cause).

Now that you can see how the parts of argument fit together, we can begin to look at unsound arguments—fallacies.