类比是一种作文（或者更常见的，是文章或演讲的一部分），其中一个想法、过程或事物通过与其他事物进行比较来解释。

扩展类比通常用于使复杂的过程或想法更容易理解。“一个很好的类比，”美国律师达德利·菲尔德·马龙说，“值得三个小时的讨论。”

“类比证明什么都不是，这是真的，”西格蒙德·弗洛伊德写道，“但他们可以让人感觉更自在。”在这篇文章中，我们研究了有效类比的特点，并考虑了在我们的写作中使用类比的价值。

类比是“从平行案例中进行推理或解释”。换句话说，类比是两个不同事物之间的比较，以突出某些相似之处。正如弗洛伊德所说，类比不能解决争论，但好的类比可能有助于澄清问题。

在下面一个有效类比的例子中，科学作家克劳迪娅·卡布依靠计算机来解释我们的大脑如何处理记忆：

Some basic facts about memory are clear. Your short-term memory is like the RAM on a computer: it records the information in front of you right now. Some of what you experience seems to evaporate--like words that go missing when you turn off your computer without hitting SAVE. But other short-term memories go through a molecular process called consolidation: they're downloaded onto the hard drive. These long-term memories, filled with past loves and losses and fears, stay dormant until you call them up. ("To Pluck a Rooted Sorrow," Newsweek, April 27, 2009)

这是否意味着人类的记忆功能在所有方面都与计算机一模一样？当然不是。就其性质而言，类比提供了一个想法或过程的简化视图，一个说明，而不是一个详细的检查。

类比与隐喻

尽管有某些相似之处，但类比与隐喻并不相同。正如Bradford Stull在《比喻语言的要素》（Longman，2002）中所观察到的那样，比喻“是一种表达两组术语之间一组相似关系的语言比喻。本质上，比喻并不要求完全识别，这是比喻的属性。它要求关系的相似性。”

比较及；对比

类比与比较和对比也不完全相同，尽管两者都是并列的解释方法。在《贝德福德读者》（贝德福德/圣马丁出版社，2008年）中，X.J.和多萝西·肯尼迪解释了这一区别：

You might show, in writing a comparison and contrast, how San Francisco is quite unlike Boston in history, climate, and predominant lifestyles, but like it in being a seaport and a city proud of its own (and neighboring) colleges. That isn't the way an analogy works. In an analogy, you yoke together two unlike things (eye and camera, the task of navigating a spacecraft and the task of sinking a putt), and all you care about is their major similarities.

最有效的类比通常是简短的，而且仅用几句话就能达到目的。这就是说，在一位才华横溢的作家手中，一个延伸的类比可以起到启发作用。例如，参见罗伯特·本切利在《给作家的建议》中关于写作和滑冰的喜剧类比

类比论证

无论是几句话还是一整篇文章，我们都应该小心，不要把它推得太远。正如我们所看到的，仅仅因为两个主题有一个或两个共同点并不意味着它们在其他方面也是一样的。当荷马·辛普森对巴特说：“儿子，女人很像冰箱，”我们可以相当肯定，逻辑的崩溃会随之而来。果然：“它们大约有六英尺高，300磅。它们会制冰，而且……嗯……哦，等等。事实上，女人更像啤酒。”这种逻辑谬误被称为类比论证或假类比论证。

类比的例子

自己判断这三个类比的有效性。

Pupils are more like oysters than sausages. The job of teaching is not to stuff them and then seal them up, but to help them open and reveal the riches within. There are pearls in each of us, if only we knew how to cultivate them with ardor and persistence. ( Sydney J. Harris, "What True Education Should Do," 1964) Think of Wikipedia's community of volunteer editors as a family of bunnies left to roam freely over an abundant green prairie. In early, fat times, their numbers grow geometrically. More bunnies consume more resources, though, and at some point, the prairie becomes depleted, and the population crashes. Instead of prairie grasses, Wikipedia's natural resource is an emotion. "There's the rush of joy that you get the first time you make an edit to Wikipedia, and you realize that 330 million people are seeing it live," says Sue Gardner, Wikimedia Foundation's executive director. In Wikipedia's early days, every new addition to the site had a roughly equal chance of surviving editors' scrutiny. Over time, though, a class system emerged; now revisions made by infrequent contributors are much likelier to be undone by élite Wikipedians. Chi also notes the rise of wiki-lawyering: for your edits to stick, you've got to learn to cite the complex laws of Wikipedia in arguments with other editors. Together, these changes have created a community not very hospitable to newcomers. Chi says, "People begin to wonder, 'Why should I contribute anymore?'"--and suddenly, like rabbits out of food, Wikipedia's population stops growing. (Farhad Manjoo, "Where Wikipedia Ends." Time, Sep. 28, 2009) The "great Argentine footballer, Diego Maradona, is not usually associated with the theory of monetary policy," Mervyn King explained to an audience in the City of London two years ago. But the player's performance for Argentina against England in the 1986 World Cup perfectly summarized modern central banking, the Bank of England's sport-loving governor added. Maradona's infamous "hand of God" goal, which should have been disallowed, reflected old-fashioned central banking, Mr. King said. It was full of mystique and "he was lucky to get away with it." But the second goal, where Maradona beat five players before scoring, even though he ran in a straight line, was an example of the modern practice. "How can you beat five players by running in a straight line? The answer is that the English defenders reacted to what they expected Maradona to do. . . . Monetary policy works in a similar way. Market interest rates react to what the central bank is expected to do." (Chris Giles, "Alone Among Governors." Financial Times. Sep. 8-9, 2007)

最后，请记住Mark Nichter的类比观察：“一个好的类比就像一把犁，可以为一个群体的联想领域准备一个新想法的种植”（人类学和国际卫生，1989年）。