What is the nature of the mind?

The mind is said to have two parts, one is called Citta and the other Manas. The inner faculty which is directly related to the sense-organs and the organs of voluntary and involuntary action and rules just like a master does over his servants (bhupavat prakritinam) is Manas. The Indriyas cannot perform their respective functions without the aid of Manas. That is why we cannot cognise any object nor can we do any work if we become inattentive.

Citta is the seat of all conceptual and perceptual ideas and their Samskaras (or Latencies) while the aforesaid Manas is the seat of SankalpanaSankalpana includes volition, imagination and physical conation or Kriti. Willing is impossible without imagination. We cannot wish to go to a certain place without first drawing a mental picture of it. After volition the Manas comes down to the external organs and performs the desired work which is Kriti or physical conation. But imagination must be preceded by previous direct knowledge of the objects. So Samskaras or subliminal impressions (of previous cognitions) are necessary for it as well as recollection or Smriti. Thus these are the functions of the Citta.

Citta and Manas are in many places used indiscriminately. According to its functions (which are cognition, conation and retention) the mind is also divided into three parts, (1) Citta-vritti or the various cognitions, (2)Manas with its Sankalpana or conation and  (3) Hridaya or the store-house of all subliminal impressions. In other words, Prakhya or cognition, Pravritti or conation (both physical and psychical) and Sthiti or retention constitute the mind. Therefore Citta or the mind is defined as consisting of two parts  of which knowing and willing as supraliminal or Pratyaya and the Samskaras are subliminal.  … (from Samkhya Catechism by Swami Hariharananda Aranya)