辩论中行之有效的方法是从理念或逻辑上去说服读者，以增强辩论的效果。可是，如果我们逻辑不清就会犯逻辑错误而影响辩论的说服力。事实上，我们几乎经常犯逻辑错误，在电视上，在报纸里，在我们平时交谈中，我们都会不知不觉地走入逻辑思维的歧途。下面的片段是来自美国作家雪莉•杰克逊的短篇小说《彩票》，作品刻意描写了村民在旧传统奴役下的愚昧。从亚当斯先生和旧传统的化身长者华纳的对话中，我们可以看看长者说话的逻辑错误，从这里我们对逻辑错误的隐蔽性和频率可见一斑。
“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.
长者华纳在上面短短的五句话里犯了五个逻辑错误，其中包括四种类型：
(1) 人身攻击：“pack of crazy fools,” he said. “listening to the young folks, nothing’s good enough for them
“人身攻击”这个逻辑错误的做法是攻击对方本人，而不是反驳对方的观点，其具体逻辑思维是如果我能证明你人不好，那我也就证明了你的观点也一样不值一提。下面两种说法犯了同样的错误：
-- 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) 滑坡效应：--- “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.”
“滑坡效应”这个逻辑错误是预测只要事情的第一步发生了(这第一步是预测者不愿意看到的)，那么最坏的也会接踵而来，就好像一个放在光滑斜坡上的物体一样，或是像多米诺骨牌一样，只要运动一旦起动就一发而不可收拾。使用这种思维方式的目的是要把事情描写到最坏的程度，这样可以鼓动人们反对起动那一块骨牌，但却不合逻辑关系。下面也是两个“滑坡效应”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”
我们所追究事情发生的原因可能只是部分与根源有关的因素，没有必然的因果关系。我们如果把这种因素看成是导致某种结果的必然原因，那么，我们就犯了“可疑原因”的逻辑错误。
下面两个例子也是“可疑原因”的逻辑错误：
-- 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) 非逻辑关系：“there’s always been a lottery,”共2页，当前第1页12
“非逻辑关系”的意思就是没有逻辑关系，也就是英文”it does not follow”的意思。再看下面两例：
-- 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.
逻辑错误通常可以分为两种：形式逻辑错误(formal fallacies)和内容逻辑错误(material fallacies)。形式逻辑错误来源于三段论推理形式的错误，这类错误违反三段论推理的原则，即推理的前提与结论之间缺乏必要的逻辑形式关系;而内容逻辑错误涉及推理的内容，即推理的前提本身不正确或不准确。上面我们分析的长者华纳的四种逻辑错误都是属于内容逻辑错误。在谈逻辑错误之前，我们必须了解逻辑的基本类型和形式。下面我们简单介绍归纳与演绎推理、三段论的推理形式。
共2页，当前第2页12