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.

参考答案:

要点：that people can behave...by their society和that they can communicate...都是The idea 的同位语从句。without resorting to an artificial code tacitly agreed upon by their society是介词短语作状语，修饰behave。tacitly agreed upon by their society是过去分词短语作定语，修饰code。without commonly accepted grammatical rules是介词短语作定语，修饰language。

译文：那种认为人们无需借助一种全社会都默认的人为制定的规范而我行我素的想法，就像认为人们可以用一种没有被普遍认可的语法规则的语言来进行交流的想法一样愚蠢。