Deductive reasoning is concerned with the general premises of the argument and a conclusion. This is an extremely logical kind of argument that should if used properly, result in solid conclusions. They have to meet strict conditions in order to be successful. This includes being able to spot poor reasoning. When used in logic, deductive reasoning is known as a syllogism. In rhetoric, it’s often called enthymeme.
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Definition of Deductive Reasoning
The words “deductive reasoning” come from the Latin meaning “leaning.” It refers to a type of argument that uses one of three forms: modus ponens, or the inference rule, modus pollens or the law of contrapositive, or syllogism.
Deductive Reasoning Rules
Also now as the law of detachment: an inference rule, also known as “modus ponens.” The conclusion is deduced from the condition and antecedent. Or, the first and second premise. This reasoning is also known as affirming the antecedent. The first premise is a conditional statement and the second affirms it. For example:
- If she was born in 1982, then she was born in the 1980s.
- She was born in 1982.
- Therefore she was born in the 1980s.
Also known as the law of contrapositive: the opposite of the law of detachment. It is based on the second premise negativing the consequent of the previous statement. For example:
- If she was born in 1982, she was born in the 1980s.
- She was not born in the 1980s.
- Therefore she was not born in 1982.
Syllogisms are always three lines long and contain a common term in both premises but not in the conclusion. There are also a few different types of syllogism. They are categorical, conditional, disjunctive, enthymemes, and syllogistic fallacy. A general example is:
- All dogs scare her.
- There is a dog.
- She is scared.
Examples of Deductive Reasoning in Literature
In Hemingway’s classic short novel, The Old Man and the Sea, there is an often-quoted passage that serves as a great example of deductive reasoning.
They sat on the Terrace, and many of the fishermen made fun of the old man, and he was not angry. Others, of the older fishermen, looked at him and were sad. But they did not show it, and they spoke politely about the current and the depths they had drifted their lines at and the steady good weather and of what they had seen.
In these lines, Hemingway uses deductive reasoning in order to depict the circumstances surrounding the old man. The other fishermen regard him differently. The young ones make fun of him while the older ones feel sorry for him, having a better understanding of who he is and what it’s like to fail.
Edgar Allan Poe’s well-loved poem ‘The Raven,’ also has a good example of deductive reasoning. The tapping on the door comes gently and mysteriously.
Once upon a midnight dreary, while I pondered, weak and weary,
Over many a quaint and curious volume of forgotten lore,
While I nodded, nearly napping, suddenly there came a tapping,
As of some one gently rapping, rapping at my chamber door.
“‘Tis some visitor,” I muttered, “tapping at my chamber door –
Only this, and nothing more.
There is only one visitor, the speaker deduces. This means that whoever is there is also responsible for the sound. The speaker has to work out what’s going on around him but he isn’t the most reliable narrator.
Explore more Edgar Allan Poe poems.
Valid and Invalid Deductive Reasoning
A valid augment is that which has an argument that must be true if the preceding premises are true and valid. But, if that’s not the case, then the argument is invalid. Knowing how to spot an invalid deductive argument is important when one is seeking out valid arguments. It can come in handy when one finds themselves in a debate. An example of an invalid deductive argument is:
- All women are mortal.
- Tom is mortal.
- Tom is a woman.
In this example, the two premises “All women are mortal” and “Tom is mortal” are true. But, they don’t equal the conclusion “Tom is a woman.” It’s important to analyze each premise of a deductive argument in order to ensure they make sense on their own, together, and lead validly to the conclusion.
Deductive and Inductive Reasoning
Deductive reasoning is also known as top-down reasoning. This sets its apart from inductive reasoning which is known as bottom-up. It uses the same kind of premises and conclusions that deductive reasoning does, but presents them in the reverse order. For example:
- Sarah and Josie are yoga instructors.
- Sarah and Josie enjoy eating out
- Therefore all yoga instructors enjoy eating out.
Inductive reasoning is used to find probable conclusions other than certain conclusions. It’s used to test theories in the real world and can lead to some false conclusions, such as the example above.
Why Do Writers Use Deductive Reasoning?
Writers use deductive reasoning in order to craft discussions, speeches, and literary works that are well thought out and believable. The written pieces should be convincing and even persuasive with deductive reasoning is used correctly. The writer should be able to convince the reader of whatever premise and details they’re working with.
Related Literary Terms
- Ad Hominem: uses irrelevant information in an attempt to discredit someone’s opinion or argument.
- Bandwagon: a persuasive style of writing that is used to convince readers of an argument or make them understand a certain perspective.
- Bias: undue favor or support to a particular person, group, race, or one argument over another.
- Concession: a literary device that occurs in argumentative writing in which one acknowledges another’s point.
- Literary Argument: the argument of a piece of literature is a statement, towards the beginning of a work, that declares what it’s going to be about.