有读者问： 有这样一句话：
(1)A definition paragraph defines, describes, or explains an unknown term by relating that which is unknown to that which is already known.
请问这里的两个that各指代什么？
困难在于不知道that which是一个比较固定的词组。这个词组比较正式或古旧，所以许多词典（包括国内出版的《新英汉词典》和《英汉大词典》）未予收入。所幸张其春等编的《简明英汉词典》（商务印书馆）倒有一个解释，说that which中的that是“关系代词的前述词（即我们语法书中所说的先行词）= the+noun”，并有一例：
(2)That which you bade me do, I did.
《简明牛津词典》（The Concise Oxford Dictionary ）也有解释，说that是一代词，代替了“the” +noun，通过其后的关系代词完成其意义，举例如：hold fast that which is good。that which的意义实际上相当于what，往往可译作“所”。所以上述句（1）中的by relating that which is unknown to that which is already known即可译为：用所已知者解说所不知者。上述句（2）亦可用“所”来译作：你所吩咐我做的事，我已做了。再如：
(3)I have that which you gave me.（意谓“我已有你所给我的东西”）
(4)He pointed out that children should have different kinds of knowledge, especially that which cannot be learned from textbooks.
（that which cannot be learned from textbooks意谓“课本中所不能学到的东西”）
(5)Real miracles never seem to be miracles, and that which at the first blush resembles one usually proves to be an instance of the extremely prosaic.
（that which at the first blush resembles one意谓“乍一看与奇迹所相似的东西”）
当然也有不能用“所”来译的情况，如上述《简明牛津词典》所举的例子，即不宜用“所”，而似应译为“好的要坚持”。
