Logic (Philosophy)

LOGIC (Philosophy)

In everyday speech, "logic" just means something like "clear thinking." But in formal terms, there are straightforward rules like these:

• If A, then B; A, therefore B.

Example: If I work 8 hours, I will be paid $120; I worked 8 hours, therefore I got paid $120.

• Similarly, if A, then B; not A, therefore not B.

Example: I always smile when I'm happy; I'm not smiling, so I must not be happy.

• A is B; C is A. Therefore C is B.

Example: All humans are mortal; Socrates is human; therefore Socrates is mortal.

• Either A is true or B is true. One of these isn't true, so the other must be.

Example: Only two people could have murdered Pat--Mike or Doug. But Doug was in Spain, therefore Mike killed Pat.

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Valid is not the same as true. The arguments are valid, so the conclusion ("therefore") is valid. But the premises (the first parts) must be true for the conclusion to be true. In the last example, if Doug wasn't in Spain, he still could have killed Pat.

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