大乘起信論義記 3.170

3.170

謂人雖就位以分優劣，真體隨人未曾增減，故云無增減也。非前乃至常恒者，顯不增減所以也。非前際生故常，非後際滅故恒也。凡位為前際，佛果為後際也。

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.