Some may ask: Since the manas (mind faculty) is also a form of consciousness, it should perceive external objects; why then, when a person is unconscious, is there no perception of the external environment at all? The explanation is as follows: The lack of perception during unconsciousness is due to the cessation of perception by the conscious mind (mano-vijñāna). However, the manas still perceives. When the manas perceives, if the conscious mind ceases to function, it remains unaware of the manas' perception. Even if the conscious mind does not cease, without the wisdom of discernment, it still cannot recognize the perceptual function of the manas. This is because the knowing of the manas must be known by the conscious mind before sentient beings can become aware of it. Sentient beings regard the knowing of the conscious mind as knowing; what the conscious mind does not know is considered unknown.
But the conscious mind not knowing does not mean the manas also does not know. Therefore, during dreamless sleep, in unconsciousness, or when the conscious mind vanishes, the knowing of the manas cannot be conveyed to the conscious mind. Consequently, the conscious mind does not know, and if the conscious mind does not know, sentient beings remain unaware. Even if the conscious mind functions normally without disappearing, when it lacks wisdom, it still cannot know the knowing of the manas. When the conscious mind lacks wisdom, it remains unaware of what mental activities the manas is engaged in or what it perceives. Thus, whether the manas possesses the self-witnessing self-awareness (svasaṃvedana-svasaṃvitti), the conscious mind cannot observe it; whether the manas knows the existence of the self, the conscious mind also cannot observe it. Therefore, when the conscious mind cannot observe these aspects, one should not make judgments about whether the manas is like this or not like that.
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