基本逻辑笔记

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Truth values In what follows we shall talk a lot about the “truth value” of a statement. By “truth value” we simply mean whether the statement is true or false. For instance, the truth value of the statement 2 is an even number is “true”, and the truth value of the statement 2 is an odd number is “false”. Logically equivalent We shall often say that two statements are logically equivalent. This will mean that the two statements have the same truth values in the same circumstances. The term “not” The term “and” The term “or” Statements of the form: “if A then B” In logic, the statement if A then B means that if A is true, then B must also be true. If A is true, then B is true. If A is false, then B may be either true or false. The only way that if A then B can be false is if A is true and B is false. if 0=1, then 2+2=5. This is a true statement, since 0=1 is false. It is   true even though 2+2=5 is false. This may seem a little strange at first   sight! ………………………………