In mystery stories, you often hear about deduction. Using deduction, detectives can solve cases from a myriad of clues. Deductive reasoning, which can work in any argument, draws a conclusion from general observations. In a crime story, a detective might look at many general facts about a crime scene and deduce the kind of crime that occurred. When it comes to deductive writing, deduction plays a strong role in argumentation and rhetoric.
Deduction: Definition of Deductive Reasoning
Deductive reasoning is one of the best ways to draw a conclusion. It will help you in argumentation, report writing, and other logical forms.
Deductive reasoning, also called deduction, narrows general facts into specific conclusions.
Deduction is highly accurate when performed properly. Here’s what that looks like.
Flowers are plants.
Sunflowers are flowers.
Therefore, sunflowers are plants.
This example of deductive reasoning takes a general fact (flowers are plants), adds a second fact (sunflowers are flowers), and draws a narrower conclusion than the general fact (sunflowers are plants).
This line of reasoning is also called a categorical syllogism.
Elements of Deductive Writing
In deductive writing, deduction requires two key ingredients: truth and logic. To demonstrate how important these things are, here are examples of deduction lacking truth and logic.
Example of Untruthful Deductive Reasoning
Take the following deductive syllogism.
TVs are electronic.
Cats are TVs.
Therefore, cats are electronic.
Obviously, something has gone wrong with this line of deductive reasoning, but what? Examine each of the premises (the first two lines).
The first premise, “TVs are electronic,” seems okay. This is true.
The second premise that “cats are TVs” is clearly incorrect. The premise is false, so the conclusion is incorrect.
This deduction contains a logical fallacy. A logical fallacy occurs when a conclusion is arrived at through faulty reasoning. This deduction specifically contains an informal logical fallacy, meaning that some part of it lacks truth.
Example of Illogical Deductive Reasoning
Take the following deductive syllogism.
Fish live in the ocean.
Dogs live on land.
Therefore, dogs live in the ocean.
The issue with this deduction is different from the last. First, both premises (the first two lines) are true. Second, notice how the two premises share no elements in common (unlike the last example). The first premise discusses fish in the ocean, while the second discusses dogs on land. Herein lies the deductive flaw of this example.
Deduction requires that the premises share elements in common. Because this example has no shared elements between the premises, you cannot draw an accurate deductive conclusion from them.
Like the example before it, this example contains a logical fallacy. However, it specifically contains a formal logical fallacy, where the issue lies in the lack of logic and not the lack of truth. This kind of error is also called a non-sequitur.
Fig. 1 - A non-sequitur is a kind of failed deduction.
Deductive Writing Example
Deductions do not always appear as three-line syllogisms. They can also be strung together in a prose essay. Here is how you might see a deductive line of reasoning appear on a timed test or in a paper or journal.
Throughout the story, Briar stays out of fights and avoids military service. Regarding his own disposition, Briar says on page 201, “Not am I a man of action. Even when I garden under the eves of Ham Manor, seldom would I choose the sharpened trowel over my blunt fingers.” Webb, ever the militant, rants several times at draft dodgers throughout the book. He says on page 113, “A man who won’t pick up a sword should best fall on one.” Given his dislike for pacifism, it’s safe to conclude that Webb has more reasons to dislike Briar than Briar’s friendly relationship with Webb’s wife.
Can you piece together the deduction in this essay paragraph? Here is how you might rewrite the deduction as a categorical syllogism.
Briar is a pacifist.
Webb doesn’t like pacifists.
Webb doesn’t like Briar.
In an essay, this piece of deductive reasoning would only be one part of a larger argument. This larger argument would be clear in a thesis statement like this one.
Webb’s deep insecurities regarding the line between war and peace radicalize him throughout the story. In the end, it is not what he sees in others but what he sees in himself that causes him to make the charge across the battlefield that ends his life. Although he begins the story thinking he will die a martyr for his country, he commits an empty suicide.
In such a context, the deduction about Briar would be one small point to help establish Webb’s radicalization. It would serve alongside many other lines of reasoning to support the thesis.
Fig. 2 - Deduction supports a thesis statement.
Induction vs. Deduction
Induction is the opposite process of deduction.
Inductive reasoning, also called induction, uses specific examples to draw a more general conclusion.
Unlike deduction, induction is not flawless when properly executed. However, it can still be useful for essay writing and logical argumentation. Here’s an example of induction.
Joan has fostered 212 cats.
Of those, 203 used the provided litter box immediately.
Therefore, most cats immediately adjust to using a litter box.
Inductive reasoning is the kind that you will often see employed in research studies. These studies survey many different people with specific results. From those specific results, researchers draw a larger conclusion about whatever it is they are testing.
As stated, though, induction is by no means flawless. If you take only one example and try to apply it to everything, that is a logical fallacy called hasty generalization.
Joan fostered a cat.
It didn’t use the provided litter box immediately.
Therefore, cats don’t immediately adjust to using a litter box.
This is a hasty generalization because the sample size is very small: one, in this case.
When using inductive reasoning, use appropriately large sample sizes.
Difference Between Inductive and Deductive Methods of Report Writing
Now that you understand the difference between deduction and induction, here is the difference between a deductive and inductive report.
A deductive report or essay contains a thesis statement. It uses many facts to draw a more specific conclusion (the thesis).
This would be like an essay for a timed test for English class.
An inductive report or essay provides an opinion or a result. It uses many instances to draw a larger conclusion about the subject.
This would be like a scientific survey or report.
In most instances of persuasive writing, you want to write a deductive essay. This deductive essay might contain inductive conclusions. For instance, you might use a research trial as evidence. However, the overall trajectory of your essay will be deductive. It will contain a rich thesis statement and attempt to narrow a diverse set of facts into specific support to back your thesis.
You will probably only write an inductive report or inductive paper if you are conducting research.
Deduction - Key Takeaways
- Deductive reasoning, also called deduction, narrows general facts into specific conclusions.
- Deduction is highly accurate when performed properly.
- Accurate deduction must be truthful and logical.
- Unlike deduction, induction is not flawless when properly executed. Use inductive essays for research papers.
- Use deductive essays for most instances of persuasive writing, such as essays for classes and timed tests.
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