Deductive Reasoning
Deductive reasoning moves from the general to the particular. It takes a general premise and deduces particular conclusions. A "valid" deductive argument is one in which the conclusion necessarily follows from the premise. (All dogs have fleas. This is a dog. Therefore this dog has fleas.) The premise may not be "true" but the form of the argument is nevertheless "valid". (If all dogs do have fleas, and if this is a dog, then this dog must necessarily have fleas.)
An "invalid" deductive argument will contain something in the conclusion wholly new and independent from those things mentioned in the premise of the argument. (If all dogs have fleas, then my dog must have ticks. But ticks are not mentioned in the premise.) Sometimes it is not so obvious that something new has been introduced in the conclusion. (Only man is a rational being. Therefore, no woman is a rational being. This argument equivocates on the meaning of "man." In the premise, the word "man" means mankind, including woman. In the conclusion, the word "woman" is used to designate that portion of mankind which is of the female gender as distinguished from the male portion called "man." So a new concept — a distinction in gender — is introduced in the conclusion.)
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