这一类文章的性质和目的是在解释，提供知识，和表明观念；“exposition”这个字的意思就是“explanation”(解释)，既然如此，这类文章最大的要求是清晰；作者可以使用任何方法和技巧，以期能达到清晰明确的境地。

　　依一般分类，说明文有下列六种：

　　(1)定义法(Definition)

　　(2)例说法(Example)

　　(3)分类法(Classification)

　　(4)分析法(Analysis)

　　(5)类比法(Analogy)

　　(6)比较法或对比法(Comparison Or Contrast) [注]说明文中亦可穿插以描写文与叙述文的写作方法。

　　说明文是写作最常用到的体裁，其最常用的方法就是将事物加以解说。例如，说明事物的过程，说明事物的因果，说明事物的状态等等。对人物来说，说明该人物的处境，说明该人物的想法，说明该人物的背景等等。如果要将描述文与说明文作一严格区分的话，那就是描述文重官能的观察与感受，而说明文重头脑的分析与理性的研判。对事物的体验是一同事，而对事物的彻底了解又是另一同事。体验是官能的，而了解却是观念上的问题。闻到花是香的，乃是官能上的，而花为什么是香的，却是知识性的观念上的问题。前者是描述文的范围，后者则是说明文的范围。

　　知识性的东西我们可从百科全书，字典辞源，各科手册，以及语言的文法修辞等书籍来获得。但这些仅是资料而已，要将资料变成一篇文章还需要文字的组织能力。我们可以这样说，说明文是一种实用的写作方法，将人类的知识系统化，这可以说是一种其功至伟的写作方法。

　　说明文最简单的方法就是例证法：将详细资料表列，一一举证，以说明什么是什么，怎样发生，结果怎样。另外两种较复杂的说明文是分析法与定义法：其方法必用到归类法与等级法。例如，要说明一件机器的操作，就必须把这件机器加以归类，引述其功能目的。又如，要了解一条鲸鱼，首先要将其归为哺乳类，因为它是胎生动物而非卵生鱼类。而后再进而分析鲸鱼中许多不同的类别。定义法以研究其事物的必然性为主，不涉及其他。

　　说明文中当有比较法与对照法，必须用此类方法的原因，乃是有些抽象事物无法具体归类，于是只好用类例来此较对照之；即以类例说出与其相关的概念，使读者容易接受其概念。

　　<范文> Dreams

　　Men have always been interested in their dreams. In superstitious (2) ages and countries, dreams were regarded very seriously. Every dream was supposed to have a meaning, and it was the business of priests and astrologers(3) to interpret people's dreams for them. They were generally looked upon as predictions of the future warnings of coming dangers or sorrows, or prophecies of coming good fortune. Dreams were believed to be supernatural communications from the gods, or the spirits of the dead, and so were divine revelations.

　　Educated people no longer look on dreams in this way. Some laugh at them as mere illusions, and not worthy of any serious consideration; but others find them interesting for psychological(4) study. For one thing, dreams prove that in the sleep we are not completely unconscious. Part of our brain is awake and working. We are unconscious of our immediate surroundings in sleep. We see nothing, we hear no-thing, and know nothing of what is going on around ns. Yet the fact that we dream proves we are not completely unconscious.

　　It is often interesting to try to discover the cause of dreams. The causes of some dreams are purely physical. A heavy supper which causes indigestion, will give us nightmare, particularly horrible and terrifying kind of dream. Some one knocking at our door may make us dream we are on the battlefield and deafened with the thunder of guns. One man dreamt he was walking near the crater of a volcano and his feet were burnt with the hot rocks; but when he woke up, he found his feet were pressed upon his hot-water bottle!

　　Most dreams are really confused and disjointed memories of past events in our lives. We can often trace a dream back, and find it was suggested by something we saw or heard or read about only a day or two before. For example, a lawyer, who had been thinking over a case late at night, dreamt when he went to bed of nothing but lizards. When he came to his study in the morning, he found that, while he was thinking out his case, he had been staring unconsciously at his clock, the case of which was decorated with (5) the bronze image of a lizard.

　　Dreams refer to the past, not the future. And they are not supernatural, but only the confused workings of our own imaginations.

　　语汇说明

　　(1)Dreams(梦)：这篇说梦的文章是从四个方面来加以述说：(A)梦曾被认为是预卜未来的超自然表现；(B)梦证明我们睡着了的时候并非全然没有知觉；(C)有些梦是因生理上的原因而引起的；(D)大部份的梦是过去事件的杂乱记忆。

　　(2)superstitious：迷信的

　　(3)astrologers：占星家；星象家

　　(4)Psychological：心理上的；心理学上的

　　(5)be decorated with：饰以…；装饰着...