成唯識論述記 4.546

4.546

述曰：彼亦自說自位之識行相、所緣微細難知，故以為因。或汝所言此位第六識，應非是實第六識攝；行相、所緣不可知故；如壽、煖等。

The Commentary states: They themselves also say that the mode of activity (ākāra) and the cognitive object (ālambana) of the consciousness in their own state are subtle and difficult to know, hence this is taken as the reason. Or: what you call the sixth consciousness in this state should not be subsumed under the real sixth consciousness; because its mode of activity and cognitive object cannot be known; like lifespan, warmth, and so on.