大乘起信論義記 3.214

3.214

問：佛身何故唯眾生識耶？答：眾生真心與諸佛體平等無二，但眾生迷自真理起於妄念，是時真如但現染相不顯其用；以彼本覺內熏妄心故有厭求，有厭求故真用即現。厭求劣故，用相即麁；厭求漸增，用亦漸細。如是漸漸乃至心源，無明既盡，厭求都息，始覺同本，用還歸體，平等平等，無二無別。未至心源已還，用於識中隨根顯現，故云識中現也。

Question: Why is the Buddha's body solely the consciousness of sentient beings? Answer: The true mind of sentient beings and the substance of all Buddhas are equal and non-dual; but sentient beings are deluded about their own true principle and give rise to deluded thoughts, at which time tathatā merely manifests defiled marks and does not display its function. Because that innate enlightenment internally perfumes the deluded mind, there is aversion and aspiration; because there is aversion and aspiration, the true function immediately manifests. Because aversion and aspiration are inferior, the marks of function are coarse; as aversion and aspiration gradually increase, the function also gradually becomes subtle. In this way, gradually reaching the source of the mind, once avidyā is exhausted and aversion and aspiration entirely cease, incipient enlightenment is identical to the innate, and function returns to the substance, equal and uniform, non-dual and non-different. Before reaching the source of the mind, the function manifests in consciousness according to the capacities [of beings]; hence it is said to manifest in consciousness.