The general name by which mind is known in the Hindu scriptural works is ‘antaḥkaraṇa,’ the ‘inner (antaḥ) instrument (karaṇa).’ Even though the external sense-organs like the eye or the ear are functioning, no cognition becomes possible unless the mind is connected with them. Hence the significance of the name.
Among the three states of consciousness viz., jāgrat (waking state), svapna (dream state) and suṣupti (deep sleep state), the antaḥkaraṇa is actively functioning in the first two and dormant in the third.
This antaḥkaraṇa functions in four different ways and gets four different names according to the functions. While functioning as the receptive and retentive faculty it is called ‘citta’ or mind-stuff; as the questioning, doubting and willing faculty, it is called ‘manas’ or mind; as the deciding faculty it is called ‘buddhi’ or intelligence; and as the identifying faculty which identifies itself with each of these functions it is called ‘ahaṅkāra’ or the ego.
These words are sometimes used to mean the antaḥkaraṇa or mind as a whole and sometimes to mean the particular faculty indicated. The sense in which they are used has to be made out from the context.