大乘起信論義記 3.170

3.170

This means that although persons are divided into superior and inferior in terms of their stations, the true substance never increases or decreases according to the person; hence it says 'without increase or decrease'. 'Not born in the prior… permanent and eternal' reveals the reason why it does not increase or decrease. It is permanent because it is not born in the prior limit, and eternal because it is not extinguished in the posterior limit. The station of ordinary persons is the prior limit, and the fruit of Buddhahood is the posterior limit.