大乘起信論裂網疏 5.63

5.63

此即以所發正直心而為方便行也。不住生死是奢摩他觀，不住涅槃是毘鉢舍那觀。止觀一心中修，乃順真如法性，故名行根本方便。

This is precisely taking the straightforward mind that was generated as the practice of skillful means. Not abiding in birth and death is śamatha contemplation; not abiding in nirvāṇa is vipaśyanā contemplation. Cultivating calming and contemplation within the one mind accords with the dharmatā of tathatā; therefore it is called the fundamental skillful means of practice.