上句见：Day271
The Story of Philosophy《哲学的故事》第1章Plato第4节IV. The Ethical Problem第6段第5句：

How does Socrates — i.e., Plato — meet the challenge of this theory? At first he does not meet it at all. He points out that justice is a relation among individuals, depending on social organization; and that in consequence it can be studied better as part of the structure of a community than as a quality of personal conduct. If, he suggests, we can picture a just state, we shall be in a better position to describe a just individual. Plato excuses himself for this digression on the score that in testing a man's vision we make him read first large type, then smaller; so he argues, it is easier to analyze justice on a large scale than on the small scale of individual behavior. But we need not be deceived: in truth the Master is patching two books together, and uses the argument as a seam. He wishes not only to discuss the problems of personal morality, but the problems of social and political reconstruction as well. He has a Utopia up his sleeve, and is resolved to produce it. It is easy to forgive him, for the digression forms the core and value of his book.
浙江大学译本：柏拉图为自己这段离题的话辩解说，我们测试一个人的视力时会让他先读大字，再读小字，因此，从一个更大更广的角度来解读正义，而不是关注小范围的个体行为，势必更容易。

解析

1、Plato excuses himself for this digression on the score that in testing a man's vision we make him read first large type, then smaller;
柏拉图为自己的离题行为辩解，理由是在测试一个人视力时，我们会先让他读大字，然后再读小字；
excuses oneself for：固定搭配，意思是“为自己……而辩解；为自己的……行为找借口”。
The lawyer, over the Camembert and Burgundy, began to excuse himself for his delay.律师一边喝卡芒贝尔奶酪一边勃艮第葡萄酒，开始为自己的延误道歉。《人与鬼》
digress: to speak or write about something that is different from the main subject being discussed. 离题
There are hardly any digressions of an episodic nature.几乎没有任何情节性质的题外话。《托尔斯泰》
on the score that: 固定搭配，表示“因为；基于……理由”

2、so he argues, it is easier to analyze justice on a large scale than on the small scale of individual behavior.
所以他认为，从大的层面分析要比从小规模的个人行为层面分析要来得更容易。

这句指出柏拉图这么研究正义的理由。测视力以大字往小的方法类比他研究正义的方法。可是现在测视力会从小字往大的测，这又怎么说呢？这有点站不住脚。

柏拉图为自己的离题行为辩解，理由是在测试一个人视力时，我们会先让他读大字，然后再读小字；所以他认为，从大的层面分析要比从小规模的个人行为层面分析要来得更容易。