成唯識論了義燈 2.140

2.140

答：定姓可得聖，雖有所知不名重，無姓不得聖，故二障名重。

Answer: Those of determinate lineage can attain nobility, so although they possess [the obstruction to] knowables, it is not termed heavy; those lacking lineage cannot attain nobility, therefore the two obstructions are termed heavy.