大乘義章 1.54

1.54

言生了者，望方便果，報佛之性。是其生因。真心體上從本已來，有可生義，生彼果故。故《涅槃》云「佛性雖無，不同㝹角雖以無量方便不可得生，佛性可生。以可生故，名為生因。」

Regarding 'generating and revealing [causes]': looking toward the fruition of skillful means, the nature of the retribution-Buddha is its generating cause. In the essence of the true mind, from the beginningless past, there is the capacity to generate, producing that fruition. Hence the Nirvāṇa [Sūtra] says: 'Although Buddha-nature is non-existent, it is not like a rabbit's horn which, even through countless skillful means, cannot be brought into being; Buddha-nature can be brought into being. Because it can be brought into being, it is called a generating cause.'