The Three Laws of Logic

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  • 0:04 The Laws of Logic Origins
  • 0:26 Law of Identity
  • 1:16 Law of Non-Contradiction
  • 1:41 Law of the Excluded Middle
  • 2:19 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.

In this lesson we will discuss the three laws of logic. Learn about each of the three laws, what they mean, and how they apply to rational thinking. Then test your knowledge with a quiz.

The Laws of Logic Origins

There are three fundamental laws upon which logic and rational thinking are based. These three laws are thought to have originated with Aristotle, who believed that the laws are necessary conditions for rational thinking to occur. The three laws are the law of identity, law of non-contradiction, and law of the excluded middle. Let's examine each of the three laws of logic in more detail.

Law of Identity

The law of identity states that if a statement has been determined to be true, then the statement is true. In formulaic terms, it states that 'X is X'. For example, if I make a statement that 'It is snowing,' and it's the truth, then the statement must be true. If we look at the law of identity in more general terms, it says that each thing that exists is made up of its own particular characteristics that are a part of what it is.

When you apply this to logic, the law of identity essentially means that everything is itself, and it cannot be something else. Snow cannot be clouds, and water cannot be a pole. Each thing is something specific that has a particular identity. So when I say that it is snowing, snowing refers to a particular event. Given that 'snowing' refers to a specific thing, if I make this statement while it's actually snowing, then it must be a true statement.

Law of Non-Contradiction

According to the law of non-contradiction, a statement such as 'It is snowing' can't be both true and false. In formulaic terms, it states that 'X is not non-X'. This means that it cannot be both snowing and not snowing during the same time period in the exact same location. In other words, nothing that is true can contradict itself. The law of non-contradiction is very important. Without it, we would not be able to think rationally.

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