There is something by reason of which man is man.

【翻译】人之所以为人，总有某种原因的。

【分析】by reason of which man is man：是定语从句修饰something。

本句的特点是，短语介词by reason of 前置于关系代词which之前。该用法在英语中十分常见。例如：

He lives in a cottage in front of which there are many trees.

他住在一所平房里，前面有许多树。

按限制性从句翻译：他住在前面种着许多树的平房里。

by reason of = because of因为；由于

主句的含义：There is something （事物）总是有某种内在的东西。something 表示事物内在的东西。

定语从句的含义：by reason of which man is man 由于此内在的东西，人才是人。

整个句子的含义是：人之所以是人（区别于其他动物），总是有某种内在原因的。

我paraphrase 这个句子：

There is something, and because of this something man is man.