The idea that people can behave naturally without resorting to an artificial code tacitly agreed upon by their society is as silly as the idea that they can communicate by using a language without commonly accepted grammatical rules.

　　參考答案:

　　要點：that people can behave...by their society和that they can communicate...都是The idea 的同位語從句。without resorting to an artificial code tacitly agreed upon by their society是介詞短語作狀語，修飾behave。tacitly agreed upon by their society是過去分詞短語作定語，修飾code。without commonly accepted grammatical rules是介詞短語作定語，修飾language。

　　譯文：那種認為人們無需借助一種全社會都默認的人為制定的規范而我行我素的想法，就像認為人們可以用一種沒有被普遍認可的語法規則的語言來進行交流的想法一樣愚蠢。