The Principle of Silent Recitation

The manas is the consciousness that acts as the master; bodily, verbal, and mental actions are all regulated and directed by the manas. The same is true of silently reciting sūtras and mantras in the mind: when the manas wants to recite silently, the mind consciousness recites inwardly without making a sound. Vocal recitation is also done by the mind consciousness — it is because the manas wants to recite that the mind consciousness recites. It is also possible that the mind consciousness wants to recite, and only when the manas deems it acceptable can the mind consciousness recite. In short, the matter of recitation is decided by the manas and carried out by the mind consciousness. 

Why does the manas choose between vocal recitation and silent recitation? This choice is extremely hidden and difficult to observe. The mind consciousness always feels that it happens automatically and naturally. This feeling shows that the mind consciousness is very coarse and lacks meditative concentration (samādhi), so it cannot observe the subtle and hidden details within. The manas makes its choice between vocal and non-vocal recitation based on the state of body and mind, and based on whether the body has sufficient internal energy (qi). When in meditative concentration, once the mind starts moving, one feels very tired; vocal recitation involves a greater amount of activity, and the mind consciousness feels fatigued, so the manas automatically chooses not to make a sound. When the body is exhausted, it will also choose to recite silently in the mind. On the surface it seems automatic, but in fact it is because the manas relies on the eighth consciousness (ālaya-vijñāna) and knows the state of body and mind extremely quickly and closely, and can make choices beneficial to body and mind very rapidly — things that many people's mind consciousness does not know about. 

Silent recitation can increase meditative concentration; or, once meditative concentration has strengthened, one can no longer manage vocal recitation — vocal recitation feels mentally exhausting — and so one automatically switches to silent recitation. When meditative concentration strengthens further, even silent recitation becomes tiring, so one switches to vajra recitation, moving only the lips without making a sound, with the sound in the mind also becoming faint. When even vajra recitation feels forced, one switches to mindful recollection in the mind. When meditative concentration strengthens further, even mindful recollection is no longer possible, and one enters deep meditative concentration, without any thoughts or mental notions at all.