1)句子的主干是：Social science is that branch of intellectual enquiry;
2)定语从句：which后面是一个定语从句，其先行词是social science;
3)方式状语：in the same reasoned， orderly， systematic， and dispassioned manner是方式状语。其中的reasoned， orderly， systematic， and dispassioned是并列定语，修饰manner;
4)定语从句：that natural scientists use for the study of natural phenomena是定语从句，其先行词是manner;
5)拆分后句子的总结构是：
Social science is that branch of intellectual enquiry(主干) which seeks to study humans and their endeavors(定语从句) in the same reasoned， orderly， systematic， and dispassioned manner(方式状语) that natural scientists use for the study of natural phenomena.(定语从句)
