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Deductive Reasoning: Examples & Definition

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  • 0:00 Defining Deductive Reasoning
  • 0:42 Syllogisms
  • 2:38 Conditional Reasoning
  • 3:50 Lesson Summary
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Lesson Transcript
Instructor: Yolanda Williams

Yolanda has taught college Psychology and Ethics, and has a doctorate of philosophy in counselor education and supervision.

Deductive reasoning involves drawing conclusions from specific statements called premises. Learn more about deductive reasoning and test your knowledge with a quiz.

Defining Deductive Reasoning

Suppose that you wanted to find a fruit to eat. You look through the refrigerator and find a celery stick, a Granny Smith, and a cup of beans. You know that neither celery nor beans are fruits. You also know that all apples are fruits, and a Granny Smith is an apple. Therefore, the Granny Smith has to be a fruit.

This is an example of syllogism, a form of deductive reasoning. Deductive reasoning is a type of logic where general statements, or premises, are used to form a specific conclusion. The other type of deductive reasoning is conditional reasoning.

Syllogisms

Syllogisms are deductive arguments that are written in the form:

A is B
C is A
Therefore, C is B

Let's take the example above. If we broke down the syllogism into premises and conclusions, we would get:

Premise: All apples are fruits.
Premise: A Granny Smith is an apple.
Conclusion: Therefore, a Granny Smith is a fruit.

According to the first premise, all items that are classified as apples are also classified as fruits. According to the second premise, Granny Smith is classified as an apple. The first premise is a general statement, while the second premise refers to a specific case. The conclusion says that a Granny Smith has to be a fruit because of its inherent properties as an apple. This deductive argument is also valid, which means that the conclusion necessarily follows from the premises. So, does a valid deductive argument mean that the premises and conclusions are true? Suppose I formed this deductive argument:

Premise: All dogs have long ears.
Premise: Puddles is a dog.
Conclusion: Therefore, Puddles has long ears.

Given the premises that all dogs have long ears and Puddles is a dog, it is logical to assume that Puddles has long ears. After all, in this example, long ears are an inherent quality of dogs. The argument is valid. Does that mean it is also true?

Not all dogs have long ears. Certain breeds, like Yorkies or pugs, have small ears. Because the conclusions are based off the premises and one of the premises is not true, it follows that the conclusion is not true, even though it is valid. You can see from this example that if one of the premises is not true, the conclusion is also not true.

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