大乘起信論義記 3.391

3.391

It is asked again: "If the Buddhas have spontaneous activity and are able to appear everywhere to benefit sentient beings, then if all sentient beings see their bodies, behold their supernatural transformations, or hear their teachings, there is none who does not gain benefit. How is it that most in the world cannot see them?" The answer states: "The dharmakāya of the Buddhas, the Tathāgatas, is equal and omnipresent. Because they are without intentional thought, it is described as spontaneous; yet it manifests solely in reliance on the minds of sentient beings. The minds of sentient beings are like a mirror: if the mirror has dirt on it, visible forms and reflections do not appear. In this way, if the minds of sentient beings have dirt, the dharmakāya does not appear."