Lesson 02.01

Three Laws of Logic




         

     Logic is the backbone of critical thinking. Logic is extremely useful for uncovering error and establishing truth. There are principles of logic and I would like to introduce you to the first three laws of logic.  These are very important.

  1. The Law of Identity
  2. The Law of Non-Contradiction
  3. The Law of Excluded Middle

     The law of identity states that A is A. An Apple is an Apple. In other words, something is what it is. If something exists, it has a nature, an essence. For example, a book has a front and back cover with pages. A car has four wheels, seats, doors, windows, etc. A tree has branches, leaves, a trunk, and roots.  This also means that anything that exists has characteristics.  We recognize what something is by observing its characteristic.  You know that a tree is a tree because you see its branches, it's leads, its trunk, etc. 
     Furthermore, if something has an identity, it has a single identity. It does not have more than one identity.  In other words, if something exists it has a set of attributes that are consistent with its own existence.  It does not have a set of attributes that are inconsistent with itself.  Therefore we can easily conclude that a cat is not a parachute. An Apple is not a race car. A tree is not a movie.
     The law of non-contradiction tells us that A cannot be both A and not A at the same time and in the same sense. In other words, something (a statement) cannot be both true and false at the same time and in the same way. We use the law of non-contradiction constantly in discussions and debates because we are naturally able to recognize when someone is contradicting himself.  If I were to tell you that yesterday I went shopping and then later I told you that yesterday I did not go shopping, you would be correct in saying there was a contradiction. A contradiction occurs when one statement excludes the possibility of another and yet both are claimed to be true. Since we know that both cannot be true, we see a contradiction.  From this principle, we can conclude that truth is not self-contradictory. This is a very important concept. Let me repeat it. Truth is not self-contradictory.  
     The law of excluded middle says that a statement is either true or false. For example, my hair is brown. It is either true or false that my hair is brown. Another example: I am pregnant. The statement is either true or false. Since I am a male, it is not possible for me to be pregnant.  Therefore, the statement is false. If I were a female, it would be possible for me to be pregnant (given normal bodily conditions). A woman is not "kind-of" pregnant. She either is or is not pregnant - there is no middle position.  The law of excluded middle is important because it helps us deal in absolutes. This is particularly important in a society where relativism is promoted and truth statements are denied.
     Please review these three laws and become familiar with them.  They are extremely important when developing critical thinking skills.  You will see them used throughout these upcoming lessons.

------------------------ Focus Points ------------------------

  1. The law of identity says that A is A, that if something exist it has a nature, a single nature.  It is what it is.
  2. The law of non-contradiction says that A cannot be both A and not A at the same time and in the same sense. Truth is not self-contradictory.
  3. The law of excluded middle says that a statement is either true or false.

 


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