成唯識論掌中樞要 2.255

2.255

何故九地障：三無礙解為一障，餘一無礙解為一障，有何意也？要速前三方辦說故，前後輕重等故，開、合不同。

Why is it that among the obstacles of the ninth bhūmi: three unhindered understandings (pratisaṃvid) constitute one obstacle, while the remaining one unhindered understanding constitutes another obstacle—what is the meaning of this? Because one must rapidly accomplish the first three before being able to expound, and because the earlier and later are equal in lightness and gravity, their division and combination differ.