请大神分析下这句子里的语法点,Urgently,he teaches her about Descartes,who doubted,and by doing so knew at least that he could doubt.

请大神分析下这句子里的语法点,
Urgently,he teaches her about Descartes,who doubted,and by doing so knew at least that he could doubt.

被修饰的词叫先行词.定语从句不同于单词作定语的情况,它需要放在被修饰的词（即先行词）之后.定语从句一般由关系代词来引导.关系代词必须放在定语从句之首.定语从句（Attributive Clauses）在句中做定语,修饰一个名词或代词,被修饰的名词,词组或代词即先行词.定语从句通常出现在先行词之后,由关系词（关系代词或关系副词）引出.
　　关系代词引导的定语从句
　　关系代词所代替的先行词是人或物的名词或代词,并在句中充当主语、宾语、定语等成分.关系代词在定语从句中作主语时,从句谓语动词的人称和数要和先行词保持一致.
who的先行词是Descartes,that修饰前面的句子