Manas as a Mediator between Senses and Self

lakṣaṇaṁ manasō jñānasyābhāvō bhāva ēva ca| sati hyātmēndriyārthānāṁ sannikarṣē na vartatē||18|| vaivṛttyānmanasō jñānaṁ sānnidhyāttacca vartatē| 

To experience (or not experience) knowing a phenomena is the result of the mind connecting (or not connecting) the objects of the senses to the ātman. [ca śā 1.18] Therefore, when the mind is functional, attention to that which can be known occurs. [ca śā 1.19a]

Notes: The mind (manas) is the interface (sannikarṣa) between the objects of the senses (indriya-artha) and the self (ātman). The mind is seen as a faculty that receives, integrates and communicates the objects of the senses from the outer world and the mental realm to the self. Without a forward motion/functioning (varta) of the mind, the self does not have the ability to experience (abhāva) the knowing (jñāna) of the characteristics of phenomena (lakṣana). Through deductive reasoning it is understood that perception exists when mind is present and does not exist when mind is not present. Therefore, a functional mind is required for knowledge (jñāna) to be brought into attention (saṁnidhya). The Vaiśeṣika Darśana of Maharṣi Kaṇāda [3.7-10] discusses a similar view and adds that when the mind connects the objects of the senses (indriya-artha) to the self (ātman) there is an alteration (anyat) of the inherent phenomena based on how the mind joins (saṁyoga), how it relates to (samavāya), and what it excludes (virodha).

aṇutvamatha caikatvaṁ dvau guṇau manasaḥ smrṛtau||19b||

Subtleness and one-ness are understood to be two characteristics of the mind.  [ca śā 1.19b]

Notes: Charaka points out that the mind (manas) is understood (smr̥ta) to have two qualities (guṇa). The first is that smallness, fineness, or its atomic nature (aṇutvam) which could best be translated as subtleness, in that it is not gross and physically visible, or perceivable by the senses themselves. The second quality of the mind is one-ness (ekatva). Based on Vaiśeṣika philosophy, the mind’s singular (eka) quality has been interpreted by many modern authors as meaning the mind tends only to perform one action at a time, or to be able to perceive only one sense at a time. In the Buddhist literature, one-ness (ekatva) is often compared/contrasted to otherness (anyatva), indicating the mind is a single sensorimotor processing faculty, not something other than a bodily-function. The Vaiśeṣika Darśana [3.11] says that effort (prayatna) [of the karmindriyas] is not simultaneous (ayaugapadya) and knowing [through the jñānendriyas] is not simultaneous, therefore the mind as perceiver and purveyor of sensory information is considered to bring a unity to what is perceived and what is known. This is understood by some to point to the individual (singular) experience of the present moment, as well as a single individual “I” present in the person [3.14]. 

From my experience, I see the one-ness or single-ness more generally. It is not that the mind can only focus on one thing, and that we need to argue for this faculty, which modern science has shown is incorrect. Instead, it is a function of the mind which allows the individual to be selectively aware of external or internal stimuli. This is often called a bottle neck- in that as a bottle has a smaller top to slow down the pouring out of its contents, the mind also is selectively determining what we pay attention to so that we are not overwhelmed with information from the external and internal world. In Yogic Psychology, this faculty of mind is utilized to train the mind to alter what it is selectively being chosen to pay attention to, the mind’s the capacity to pay attention, and the mind’s ability to alter attention at one’s own will (not under autonomic nervous system randomness).