摩訶止觀 7.8

7.8

又諸師判位遼遠，初心行人尚未斷見，何由超過五百由旬乎？今論由旬，有橫通塞、有竪通塞。橫者具約三法：苦集為塞，道滅為通；無明十二因緣為塞，無明滅為通；六蔽覆心為塞，六度為通。竪通塞者：見思分段生死為塞，從假入空觀為通；無知方便生死為塞，從空入假觀為通；無明因緣生死等為塞，中道正觀為通。

Furthermore, the ranks determined by the various masters are remote: beginners have not even severed views yet—how could they surpass the five hundred yojanas? Now, discussing yojanas involves both horizontal obstruction and opening and vertical obstruction and opening. The horizontal fully pertains to three dharmas: suffering and origination are obstruction, while path and cessation are opening; avidyā in the twelve links of conditioned arising is obstruction, while the cessation of avidyā is opening; the six veils obscuring the mind are obstruction, while the six perfections are opening. As for vertical obstruction and opening: view and thought [delusions] and fragmented birth-and-death are obstruction, while contemplation entering emptiness from provisionality is opening; ignorance and expedient birth-and-death are obstruction, while contemplation entering provisionality from emptiness is opening; avidyā, conditioned-arising birth-and-death, and so forth are obstruction, while the correct contemplation of the middle way is opening.