大乘起信論裂網疏 3.9

3.9

問：譬如迷方，須人指示，眾生無始已來從未曾悟，則最先一佛仗誰指示？答：約真如門，尚無成佛與不成佛，何有先後？約生滅門，則譬如曠野迷方，無人可問，但當諦觀日影去來之相便識東西；眾生亦爾。若能諦觀生老病死之相便能覺悟。故紫柏大師云：「最先一佛以苦諦為師」也。

Question: As in the metaphor of being lost about directions, one requires someone to point them out; sentient beings have never awakened from beginningless time, so whom did the very first Buddha rely on to point them out? Answer: In terms of the aspect of tathatā, there is not yet even attaining Buddhahood or not attaining Buddhahood, so how could there be earlier and later? In terms of the aspect of arising and ceasing, it is like being lost regarding directions in a vast wilderness where there is no one to ask: one need only carefully observe the movements of the sun's shadow to recognize east and west; sentient beings are also like this. If they can carefully observe the marks of birth, old age, sickness, and death, they will be able to awaken. Therefore, Great Master Zibo said: 'The very first Buddha took the truth of suffering as his teacher.'