There has always been some controversy as to the authenticity of the Surangama Sutra, since no Indian version of the text has ever been discovered, and the only existing copies are in Chinese. However, it was influential in the development of Chan in China over the centuries and is particularly valued for its elaboration of samadhi and techniques of emptiness meditation that are available to everyone. This short extract talks about the starting point of meditative practice being the dropping down of basic ignorance, the way that unstirred mud will eventually fall to the bottom of a container, leaving the water inside pure and clear.
Ananda, if you wish to bring your seeing, hearing, feeling and knowing into line with the Tathagata’s absolute Eternity, Bliss, Self and Purity, you should first pick out the root of birth and death, and turn its worldly falseness back to its unworldly profound nature until it is subdued and reverts to Basic Bodhi, and then use this pure nature as the causal mind-ground (i.e. as the point of departure) to perfect your practice and realization of the fruit-ground. This is like purifying muddy water in a clean container; left unshaken in complete calmness, the sand and mud will sink to the bottom. When the clear water appears, this is called the first suppression of the intruding evil element of passion. When the mud has been removed leaving behind only the clear water, this is called the permanent cutting off of basic ignorance. Enlightenment is (pure and) unmixed; and its manifestations are not of the nature of klesa*, but are in accord with the immaculate virtues of Nirvana.
*trouble, defilement, passion