What was the social status of the anonymous accountants who produced this breakthrough? The immense volume of clay tablets unearthed in the ruins of the Sumerian temples where the accounts were kept suggests a social differentiation within the scribal class, with a virtual army of lower-ranking tabulators performing the monotonous job of tallying commodities. We can only speculate as to how high or low the inventors of true numerals were in the scribal hierarchy, but it stands to reason that this laborsaving innovation would have been the brainchild of the lower-ranking types whose drudgery is eased.

请问老师，这里的would have been是什么用法？

我的理解：这是从现在的立场对现实可能性进行推测时，情态动词的过去形式并不表示过去，而只表示说话人推测时的信心小于相应的现在形式。对什么时候的情况进行推测由情态动词后的不定式形式体现。这是是对过去情况进行推测，情态动词+have done; 

请问老师，为什么不是说话人站在过去的立场，对过去的过去的现实可能性进行推测呢？

我的理解：

由于上下文语境没有过去时间，而是 it stands to reason 所以这种推测不是说话人站在过去的立场，对过去的过去的推测。

His suitcase was not there. He might have left. 这句话是对过去的过去的推测。因为有明确的was表示过去， left发生在was not there这个谓语动作之前。所以用不定式的过去完成体。

不知道上述理解是否正确。