大乘起信論義記 3.129

3.129

若獨內因不假外緣，可如所責。然今外假用熏，及內正因方得成辨，故致前後，不可一時也。是故上開二熏習，不云一也。

If there were solely an internal cause without relying on external conditions, it would indeed be as you objected. However, now they are accomplished only by externally relying on the perfuming of function and internally on the correct cause; therefore, prior and posterior occur, and it cannot happen simultaneously. For this reason, the two types of perfuming were distinguished above, rather than speaking of only one.