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##### 解析西方逻辑

The term non sequitur literally means "it does not follow". In this section we describe fallacies which occur as a consequence of invalid arguments.

² Affirming the Consequent

Definition: Any argument of the following form is invalid: If A then B B Therefore, A

Examples: (i) If I am in Calgary, then I am in Alberta. I am in Alberta, thus, I am in Calgary. (Of course, even though the premises are true, I might be in Edmonton, Alberta.) (ii) If the mill were polluting the river then we would see an increase in fish deaths. And fish deaths have increased. Thus, the mill is polluting the river.

Proof: Show that even though the premises are true, the conclusion could be false. In general, show that B might be a consequence of something other than A. For example, the fish deaths might be caused by pesticide run-off, and not the mill. (Barker: 69, Cedarblom and Paulsen: 24, Copi and Cohen: 241)

² Denying the Antecedent

Definition: Any argument of the following form is invalid: If A then B Not A Therefore, Not B

Examples: (i) If you get hit by a car when you are six then you will die young. But you were not hit by a car when you were six. Thus you will not die young. (Of course, you could be hit by a train at age seven.) (ii) If I am in Calgary then I am in Alberta. I am not in Calgary, thus, I am not in Alberta.

Proof: Show that even though the premises are true, the conclusion may be false. In particular, show that the consequence B may occur even though A does not occur. (Barker: 69, Cedarblom and Paulsen: 26, Copi and Cohen: 241)

² Inconsistency

Definition: The author asserts more than one proposition such that the propositions cannot all be true. In such a case, the propositions may be contradictories or they may be contraries.

Examples: (i) Montreal is about 200 km from Ottawa, while Toronto is 400 km from Ottawa. Toronto is closer to Ottawa than Montreal. (ii) John is taller than Jake, and Jake is taller than Fred, while Fred is taller than John.

Proof: Assume that one of the statements is true, and then use it as a premise to show that one of the other statements is false. (Barker: 157)

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