Deductive Reasoning

Deductive reasoning is defined as a way of building an argument from general premises to a conclusion. If the principle selected is correct and clear, the rules of deductive reasoning are prepared. It is also called a top to down thinking or taking general statements to specific one sand then to a conclusion. For example, if B = C and C = D, then obviously B = D. First two premises are general while the third conclusion is specific.

Deductive reasoning is a rhetorical device rather than a literary device. However, it is used in literature as well as philosophy to build arguments.

Rules for Deductive Reasoning

There are three major rules in deductive reasoning.

  1. The first is modus ponens, which means “law of detachment.” This is inference rule in which conclusion is deduced from the first premise that is condition and second premise that is antecedent.
  2. The second rule is modus tollens, which is called “law of contrapositive”. It is based on the first premise as conditional with the second premise as a negation of the result followed by conclusion deduced from them.
  3. The third law is that of the syllogism, which takes two conditionals and then forms a conclusion.

Rule-Based Examples

Premise 1: Johnson is a student.
Premise 2: All students are young men.
Conclusion: Johnson is a young man.

Premise 1: If it rains, it means that the sky is cloudy.
Premise 2: There is not any cloud in the sky.
Conclusion: Therefore, it means that it is raining.

Premise 1: If it is a York, it means it is a dog.
Premise 2: If it is a dog, it is also a mammal.
Conclusion: If the animal is a York, it means that it is a mammal, too.

Common Examples

  1. If you want to find peace of mind, you need to identify what you can control and what you cannot. However, if others think what is not under your control is, it is their thinking. Therefore, you need not worry about what others think about you.
  2. When there are two people in this room; Jonny and his brother and you know that both do not wear spectacles, it means that Jonny also does not wear spectacles. Therefore, both have good eyesight.
  3. If there is an after two prime number and seven is also a prime number, it means that all odd numbers between two to eight are prime numbers. (Adopted from Patrick Hurley’s Concise Introduction to Logic)

Examples from Literature

Example #1

Animal Farm by George Orwell

“A bird’s wing, comrades,” he said, “is an organ of propulsion and not of manipulation. It should therefore be regarded as a leg. The distinguishing mark of man is the HAND, the instrument with which he does all his mischief.”

The birds did not understand Snowball’s long words, but they accepted his explanation, and all the humbler animals set to work to learn the new maxim by heart. FOUR LEGS GOOD, TWO LEGS BAD, was inscribed on the end wall of the barn, above the Seven Commandments and in bigger letters. When they had once got it by heart, the sheep developed a great liking for this maxim, and often as they lay in the field they would all start bleating “Four legs good, two legs bad! Four legs good, two legs bad!” and keep it up for hours on end, never growing tired of it.”

This paragraph from Animal Farm is an excellent example of deductive reasoning. Snowball is trying to prove that if wings are like legs, it means they are not hands. If they are not hands, it means that birds have four legs. Therefore, they have not two legs but four legs. Through this deductive reasoning, Snowball has proven that birds are actually animals and not human beings.

Example #2

The Old Man and the Sea by Ernest Hemingway

“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.”

This paragraph from The Old Man and the Sea is also one of the best examples of deductive reasoning. It shows the logic that as the fishermen make fun of the old man, and yet the old man is not angry. It means Santiago is not angry as he is also a man. Conversely, it also shows that as the older men do not show their anger and they are very patient.

Example #3

The Raven by Thomas Hardy

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.”

This stanza from “The Raven” also shows an example of deductive reasoning used in literature. It shows that when there is tapping on the door, and it is also gentle, it means that there is only one visitor. It also means that there is no other source of tapping except that visitor.

Functions of Deductive Reasoning

Deductive reasoning proves highly useful during discussions, speeches, writings, and literary pieces. They are also included in logical and argumentative novels or plays. The written pieces often become very persuasive and convincing when constructed with deductive reasoning. The use of deductive reasoning makes it easy to convince the audiences, using general examples to reach a specific point. Moreover, deductive reasoning allows the writing and speaking clear, rhetorical, and effective. It removes ambiguities and confusions in the arguments and helps a person become a fluent and eloquent speaker and flawless writer.