Consider this hypothetical plea from Zach to his father: “Dad, could you loan me money for gas until I get my paycheck at the end of the week? If you do, I’ll be able to haul your junk pile to the dump as well as drive myself back and forth to work. I’ll pay you back as soon as I get my check!”
Logic is based on either inductive or deductive reasoning. Understanding these types of logic can help us test the soundness of arguments, both our own and those of others.
Inductive Reasoning
You likely use inductive reasoning every day. By this kind of logic, we form conclusions based on samples. Lab experiments, for example, must be repeatable in order for scientists to gather a convincing amount of data to prove a hypothesis. If a scientist hypothesizes that addiction to a particular drug causes a certain, predictable behavior, the experiment must be carefully controlled, and must be repeated hundreds of times in order to prove that the behavior is consistently associated with the addiction and that other possible causes of that behavior have been ruled out. If we observe enough examples of an event occurring under similar circumstances, we can employ inductive reasoning to draw a conclusion about the pattern. For example, if we pay less each time we buy apples at Supermart than when we purchase apples at Pete’s Grocery, we will likely conclude, inductively, that apples are less expensive at Supermart.
Literary argument is often based on inductive reasoning. Here are two illustrations of such reasoning:
- In Robert Frost’s sonnet “Design,” the color white is used ironically to suggest that only a devious designer would clothe the universe’s evil in so much beauty. The “dimpled spider, fat and white”; the “white heal-all” flower that “hold[s] up” the moth for the spider’s feast; and the rhyming of “blight” with “white” and “right” work together to generate the poem’s disturbing sense that the innocence implied in the whiteness of the natural scene is deceptive.
- As powerful evidence of the irreversible destruction of war, Hemingway’s The Sun Also Rises presents Jake Barnes’s struggles to overcome the damage incurred during his service as a soldier in World War I. Jake’s difficulty coping with his injury, his tendency to self-medicate with alcohol, his inability to pray, and his failure to sustain an intimate relationship with another person all exemplify the terrible destruction inflicted on him by the war.
When writing a literary analysis essay, such as the paper that might develop from the second argument above, you will need to provide enough examples to support your assertion that the pattern you observed in the text does, indeed, exist.
Deductive Reasoning
Deductive reasoning, on the other hand, is drawing a conclusion based on a logical equation. It can be argued that we see deduction in its purest form in the context of scientific or mathematical reasoning. A logical equation of this sort is based on a proven assumption and/or clearly and inflexibly-defined terms. Commonly manifesting these conditions, computer programs accomplish tasks through deductive logic. For example, Mary, an art major who has completed 24 credit hours, cannot register for English 4500. This statement is true because the university course enrollment system is governed by the following logic: Only English majors with 30 or more credit hours may register for English 4500. Students who have not officially declared as English majors and/or whose records do not exhibit completion of credit hours equal to or greater than 30 will be automatically prevented from registering for English 4500. Similarly, the following statement is based on deductive logic: Glyptol paint cannot be cleaned up with water only. This conclusion is based on the fact that Glyptol contains alkyds, which are not water soluble. Therefore, clean-up of any paint containing alkyds will require turpentine or another petroleum-based solvent.
Having examined deductive reasoning in its pure form, however, we can see that argument will rarely be required in such a context. Investigation may be required in order to determine the characteristic and/ or definition of a material, but once the facts are ascertained, scientists will not need to debate whether or not alkyds are water-soluble. Persuasion becomes relevant when the issue moves beyond proven facts. As we explore issues of ethics and values, logical reasoning can seem a bit mushy, yet rather than throw up their hands in abandonment of deductive reasoning, humanities scholars generally work hard to establish valid assumptions, or generally agreed-upon notions, that can be used to help humans move closer to reasonable, or logical, social and political beliefs and behaviors.
Sir Arthur Conan Doyle’s famous fictional detective Sherlock Holmes is famous for employing deductive reasoning to solve mysteries. In “Five Orange Pips,” Holmes uses deductive reasoning to work as far as possible toward solving John Openshaw’s case, based on the facts Holmes and Watson have been given:
Now let us consider the situation and see what may be deduced from it. In the first place, we may start with a strong presumption that Colonel Openshaw had some very strong reason for leaving America. Men at his time of life do not change all their habits and exchange willingly the charming climate of Florida for the lonely life of an English provincial town. His extreme love of solitude in England suggests the idea that he was in fear of someone or something, so we may assume as a working hypothesis that it was fear of someone or something which drove him from America. As to what it was he feared, we can only deduce that by considering the formidable letters which were received by himself and his successors. Did you remark the postmarks of those letters?
Equations such as the ones being forwarded by Holmes, when seen in their complete form, comprise a three part logical statement called a syllogism.
The statement includes
- A general statement, or major premise: Middle-aged men do not readily embrace change.
- A minor premise: Colonel Openshaw is a middle-aged man.
- And a conclusion: Colonel Openshaw would not have changed his circumstances without a strong impetus.
Although some might argue that Holmes came to the major premise through inductive reasoning (observing the behavior of many, many middle-aged men), in the above passage, he asserts the major premise as the basis for his deductive logical equation proving that Colonel Openshaw must have had a strong impetus for leaving America. If we agree with Holmes’s premise, we are likely to trust his conclusion.
Here is a more questionable logical equation, considered by the characters of Stephen Crane’s short story “The Open Boat”:
If I am going to be drowned—if I am going to be drowned—if I am going to be drowned, why, in the name of the seven mad gods who rule the sea, was I allowed to come thus far and contemplate sand and trees? Was I brought here merely to have my nose dragged away as I was about to nibble the sacred cheese of life? It is preposterous. If this old ninny-woman, Fate, cannot do better than this, she should be deprived of the management of men’s fortunes. She is an old hen who knows not her intention. If she has decided to drown me, why did she not do it in the beginning and save me all this trouble. The whole affair is absurd.... But, no, she cannot mean to drown me. She dare not drown me. She cannot drown me. Not after all this work.
The following syllogism reflects the men’s attitude:
- Major premise: The world is just and reasonable.
- Minor premise: All of the men in the life-boat are good men and are working hard to survive.
- Conclusion: All of the men should survive.
You can see in the latter example that some times syllogisms are flawed, or illogical. Most of us doubt, at least some of the time, that the universe is indeed just and reasonable, at least by human standards. If it is not just, then the men’s conclusion that they ought to survive may be incorrect.
The occurrence of flawed logic is most problematic when we find an equation in its incomplete form: an enthymeme. If we consider the major premise as the underlying assumption, and recognize that this premise often goes unstated, we see that the enthymeme is a type of elliptical statement that sometimes “leaps” to its conclusion unreasonably. Consider the following examples:
Example \(\PageIndex{1}\)
- Enthymeme: Sarah, don’t eat that beef; it came from Bob’s Café.
- Major premise: All food from Bob’s Café is bad.
- Minor premise: That beef came from Bob’s Café.
- Conclusion: That beef is bad.
Example \(\PageIndex{2}\)
- Enthymeme: Lisa Harmon would be a good hire for our company; she has a degree from Harvard University.
- Major premise: Anyone with a degree from Harvard University would be a good employee for our company.
- Minor premise: Lisa has a degree from Harvard University.
- Conclusion: Lisa would be a good employee for our company.
Example \(\PageIndex{3}\)
- Enthymeme: Kill Bill is a chick flick.
- Major premise: Any movie that features a female protagonist is a chick flick.
- Minor premise: Kill Bill features a female protagonist.
- Conclusion: Kill Bill is a chick flick.
As you test the major premises above, how do they fare? Are they true? If a writer asserted the above enthymemes, would you, as the reader, agree with the major premise of each? Why or why not? What is the danger of reading (or hearing) only the enthymeme and not testing the underlying (unspoken) assumption which would complete the syllogism?