大乘起信論義記 1.133

1.133

釋中三：初體者，真性深廣，凡聖染淨皆以為依，故受大名。隨流加染而不增，返流除染而不減；又返流加淨不增，隨流闕淨不減，良以染淨之所不虧，始終之所不易，故云平等不增減也。

In the explanation there are three: First, regarding "essence," the true nature is deep and vast, relied upon by both ordinary beings and sages, defilement and purity; hence it receives the name "great." Following the flow and adding defilements, it does not increase; reversing the flow and eliminating defilements, it does not decrease. Moreover, reversing the flow and adding purity, it does not increase; following the flow and lacking purity, it does not decrease. Truly, because defilement and purity cannot diminish it, and beginning and end cannot alter it, it is said to be "equal, neither increasing nor decreasing."