大乘起信論義記 3.447

3.447

The first part has three [subdivisions]: first, establishing [it]; next, from “which means that all” downward, explaining and revealing its marks; and finally, from “this is called” downward, clarifying its name. Regarding the single-practice samādhi, as the *Mañjuśrīprajñā Sūtra* says: “What is called the single-practice samādhi? The Buddha said: ‘The dharmadhātu has a single mark; focusing one's conditions upon the dharmadhātu is called the single-practice samādhi. One who enters the single-practice samādhi completely knows the undifferentiated mark of the dharmadhātus of Buddhas as numerous as the sands of the Ganges,’” and so forth at length. Because this tathatā-samādhi can give rise to these immeasurable samādhis, it is called the root of samādhi.