If a theory were not simpler than the phenomena it was designed to model, it would serve no purpose. Like a portrait, it can represent only a part of the subject it pictures. This part it exaggerates, if only because it leaves out the rest. Its simplicity is its virtue, provided the aspect it portrays be that which we wish to study. If, on the other hand, our concern is an aspect of nature which a particular theory leaves out of account, then that theory is for us not wrong but simply irrelevant. For example, if we would analyse the stagnation of traffic in the streets, to take into account the behavior of the elementary particles that make up the engine, the body, the tires, and the driver of each automobile, however “fundamental” the physicists like to call those particles, would be useless even if it were not insuperably difficult. The quantum theory of individual particles is not wrong in studies of the deformation of large samples of air; it is simply a model of something else, something irrelevant to matter in gross.

第1句用了虚拟语气表与事实不符的假设。这相当于同时道出了对事实的肯定，即“a theory is ALWAYS simpler than the phenomena it is designed to model”。如果又出个选择题，这一选项是符合原文意思的（尽管原文没有正面叙述），专门考你虑拟语气在条件从句中的应用了。

第3句是个啥结构呢？开头其实是个倒桩，正常语序是It exaggerates this part。碰到代词我们要先搞清楚它是谁：It是前面的a theory；this part是指前面的a part of the subject it (the portrait) can represent。后面if only那部分中的it还是指theory，而the rest是指the rest of the portrait。第4句继续一大堆代词，注意先对应好的话，其余的意思不难理解正确。大量代词的句群其实是组成了一个言简意赅的语文风格，能够突显逻辑骨架。但大量代词不能无穷延续；在一个合适的时候重新正面提及对象，也会有一种“找补回来”的平衡感。这种做法就发生在了第5句。而且第4句和第5句又有联系。因为第4句用了provided that，加了一个条件；而第5句就是恰好解释如果这个条件不满足的话会怎样。因此这两够句也要连起来翻译。

第6句开始是For example了。如果是做GRE阅读我们一般看到For example就可以跳过不看，因为它无非是讲一大堆故事来重复前文已经总结的观点，观点是啥你看懂了就不影响做题了，除非有的题目问题到了这个example的细节。但现在我们是来赏析Truesdell大神的文采，并学习他的物理学思想，那么在这个For example的部分中我们的任务就是找出这个example如何体现之前总结的观点。什么观点呢？那就是第4句中的”not wrong but simply irrelevant”这件事，即如果理论没被用在它本来意图描述的方面，那它与事实不符不是“错误”而只是与事实“无关”。后面的例子具体举的是，想要为交通堵塞建模，却从基本粒子出发，可想而知就算数学上搞得定，也是无用的。但这不能总结为“量子力学是错的”；只能说“量子力学的设计目的不是为宏观物质建模”。

事实上，到这里为止的文字，都是对上一段（第4段）第一句中的“keener appraisal of the role a theory is to play”的展开解释。那么如果文后出一道题问以下哪项是作者所说的“keener appraisal of the role a theory is to play”，然后从全文不同地方截一些似是而非的段落作答案选项，你都可以不用具体看这些选项，只需要对一下我是从全文哪些地方截的，只有从第5段截的才有可能是对的，快速排出一些连截的地方都不对的选项——阅读题就是这样快速做掉的。

而且我们可以看到，第6段开头的“With this sober and critical understanding of what a theory is”，其实就是重复第4段的“keener appraisal of the role a theory is to play”。这里，keener这个词其实是较为笼统的。所以如果我又出道题：以下选项与第4段中的keener一词意义相符的是，然后除了“sober and critical”这一项之外，再找几个文中出现过的形容词搞三个选项，你要意识到由于sober and critical这组恰好出在对原句的复述句上所以它就是（出题者认为的）正确答案，考你是否看得出文章的呼应结构。这种题都是阅读题中的难题，把那些只顾得上一个一个单词看懂，顾不上全文把握的考生给刷下去。

With this sober and critical understanding of what a theory is, we need not see any philosophical conflict between two theories, one of which represents a gas as a plenum, the other as a numerous assembly of punctual masses. According to the physicists, a real gas such as air or hydrogen is neither of these, nothing so simple. Models of either kind represent aspects of real gases; if they represent those properly, they should entail many of the same conclusions, though of course not all.

这一段总体是又举了一个例，重复前面的观点，但强调的角度不同，并暗暗引出了后文还会继续讲到的“不同的理论如何比较”的问题。