大方廣圓覺修多羅了義經略疏 2.103

2.103

根本無明也。不達法界性相，是礙正知見義故。故彼論云：「是心從本已來自性清淨，以不達一法界故，心不相應，忽然念起，名為無明。」

This is fundamental avidyā. Because failing to penetrate the nature and characteristics of the dharmadhātu obstructs right knowledge and vision. Thus that treatise says: 'This mind from the beginning is inherently pure in its self-nature, but because of not penetrating the one dharmadhātu, the mind does not correspond, and suddenly thoughts arise, which is called avidyā.'