《汉书·志·艺文志》
大概是有原因就容易一些，没有原因就困难一点，所以依照旧书来编排数术为六种。
盖有因而成易，无因而成难，故因旧书以序数术为六种。
Probably when there is a reason, it is easy; when there is no reason, it is difficult. Thus, based on the ancient books, the number arts were classified into six types.