成唯識論演祕 4.90

4.90

釋曰：三性三受並皆易了，從初受生乃至老時，此分別意展轉而起。故言後後位轉，有二意識轉等者，謂異熟體有情本事，不待今時加行而轉無記意識，及可了知所緣行相樂、苦受等相應意識，是二意識。

The exposition states: The three natures and three feelings are all easy to understand; from initial rebirth up to old age, this conceptual mind arises successively. Hence it says 'operates in subsequent states'. 'There would be two mental consciousnesses operating and so on' means: the retribution entity, which is the basic matter of a sentient being, is the non-defiled neutral mental consciousness operating without depending on current preparatory effort, and the mental consciousness associated with pleasant, painful, and other feelings whose object and aspect are discernible—these are two mental consciousnesses.