關於辯論與邏輯

辯論中行之有效的方法是從理念或邏輯上去説服讀者，以增強辯論的效果。可是，如果我們邏輯不清就會犯邏輯錯誤而影響辯論的説服力。事實上，我們幾乎經常犯邏輯錯誤，在電視上，在報紙裏，在我們平時交談中，我們都會不知不覺地走入邏輯思維的歧途。下面的片段是來自美國作家雪莉•傑克遜的短篇小説《彩票》，作品刻意描寫了村民在舊傳統奴役下的愚昧。從亞當斯先生和舊傳統的化身長者華納的對話中，我們可以看看長者説話的邏輯錯誤，從這裏我們對邏輯錯誤的隱蔽性和頻率可見一斑。

“they do say,” mr. adams said to old man warner, who stood next to him, “that over in the north village they’re talking of giving up the lottery.”

old man warner snorted. “pack of crazy fools,” he said. “listening to the young folks, nothing’s good enough for them. next thing you know, they’ll be wanting to go back to living in caves, nobody work any more, live that way for a while. used to be saying about ‘lottery in june, corn be heavy soon.’ first thing you know, we’d all be eating stewed chickweed and acorns. there’s always been a lottery,” he added petulantly.

長者華納在上面短短的五句話裏犯了五個邏輯錯誤，其中包括四種類型：

(1) 人身攻擊：“pack of crazy fools,” he said. “listening to the young folks, nothing’s good enough for them

“人身攻擊”這個邏輯錯誤的做法是攻擊對方本人，而不是反駁對方的觀點，其具體邏輯思維是如果我能證明你人不好，那我也就證明了你的觀點也一樣不值一提。下面兩種説法犯了同樣的錯誤：

-- a critic is a crippled coach who teaches running.

-- dr. brown’s counseling on marriage can’t be worth much. he’s been divorced twice.

(2) 滑坡效應：--- “next thing you know, they’ll be wanting to go back to living in caves” --- “first thing you know, we’d all be eating stewed chickweed and acorns.”

“滑坡效應”這個邏輯錯誤是預測只要事情的第一步發生了(這第一步是預測者不願意看到的)，那麼最壞的也會接踵而來，就好像一個放在光滑斜坡上的物體一樣，或是像多米諾骨牌一樣，只要運動一旦起動就一發而不可收拾。使用這種思維方式的目的是要把事情描寫到最壞的程度，這樣可以鼓動人們反對起動那一塊骨牌，但卻不合邏輯關係。下面也是兩個“滑坡效應”的例子：

-- (about “no-smoking tables” in restaurants) --- perhaps those in power are aware that if the new yorker was compelled to deal with just one more factor in deciding on a restaurant, there would be a mass return to home cooking.

-- if cigarette ads are banned now, ads for other products such as fast cars, liquor, beer, etc. will be banned before long.

(3) 可疑原因：“lottery in june, corn be heavy soon”

我們所追究事情發生的原因可能只是部分與根源有關的因素，沒有必然的因果關係。我們如果把這種因素看成是導致某種結果的必然原因，那麼，我們就犯了“可疑原因”的邏輯錯誤。

下面兩個例子也是“可疑原因”的邏輯錯誤：

-- a research study demonstrated that children who watched my two dads rather than cheers made higher grades in school. so my two dads must be more educational than cheers.

-- the number of juveniles arrested for serious and violent crimes in the united states increased 1600 percent between the years 1952 and 1872. since this is the very period in which television became ascendant in the lives of american children, and since the programs children watched were saturated with crime and destruction, it is reasonable to argue that television was responsible for the skyrocketing increase in juvenile crime.

(4) 非邏輯關係：“there’s always been a lottery,”

“非邏輯關係”的意思就是沒有邏輯關係，也就是英文”it does not follow”的意思。再看下面兩例：

-- cindy lives across the street from her cousin. consequently the two have developed a warm relationship.

-- marijuana should be legalized. one of the reasons is that it serves as a pain killer for headaches, cramps, and other routine ailments, and has been used as such for thousands of years.

邏輯錯誤通常可以分為兩種：形式邏輯錯誤(formal fallacies)和內容邏輯錯誤(material fallacies)。形式邏輯錯誤來源於三段論推理形式的錯誤，這類錯誤違反三段論推理的原則，即推理的前提與結論之間缺乏必要的邏輯形式關係;而內容邏輯錯誤涉及推理的內容，即推理的前提本身不正確或不準確。上面我們分析的長者華納的四種邏輯錯誤都是屬於內容邏輯錯誤。在談邏輯錯誤之前，我們必須瞭解邏輯的基本類型和形式。下面我們簡單介紹歸納與演繹推理、三段論的推理形式。