I have written about this before but I see it as important enough to repeat. Bear with me.
Sigmund Freud identified 3 layers of the mind, and that hypothesis has been largely adhered to till this day. He divided mental functions into instinct, reason and morality, each governed by the id, the ego and the superego respectively. In addition to Freud's label, the word ego is also applied to the identification of self and the associated selfish behaviour exhibited by people.
A more recent study has discovered a profound bidirectional communication between the brain and the digestive system via the vagus nerve. This explains the "gut feeling" that people often get, which has been associated with the system of instinct that we have observed in animals.
I have learned from the Tripurashakthi Upanishad about the antahkarana (inner instrument), explaining the four component nature of the mind - manas, buddhi, ahamkara and chitta. To explain them, I have equated them to our current understanding of how the mind works, using a computer model as a reference.
Manas is the memory bank that stores the input received from all the senses as well as records of outputs generated by the mind. Memory is the best evidence that the mind has only a physical base. When a person totally loses his memory, the mind goes blank. Without memory, we have no mind.
Buddhi is the intellect, the thinking part of the mind which uses the information stored in manas as well as fresh input coming in from the senses. The rules for processing the information were also learned from the senses and stored in manas. These rules/guidelines (algorithms) are used to make decisions on what to do with the information and how to do it.
Ahamkara is the ego, the seat of personality, the source of self-identity. Ahamkara is governed by emotion and is subject to pride, envy, despair and so on. Ahamkara usually directs the buddhi on the decisions to be made, often overriding logical conclusions and taking rash, emotional actions. Ahamkara may be residing within the vagus nerve system.
The above three functions are easily understood and equated with our understanding of the human mind as well as computer systems (for the first two functions). For those who believe there is no afterlife, the above three are sufficient to explain what our mind does. There is no need for more.
Chitta is explained as the consciousness that comes from beyond the material realm of spacetime. It does not actively participate in our actions and remains the silent observer, collecting a record of all the decisions made by the buddhi. This record is carried beyond the death of the body into the afterlife.
Buddhi, manas and ahamkara are formed in the brain of the person, are of this material realm and expire with the death of the body. The true Self, the chitta, was never created, is never destroyed and continues into the subsequent stages of our existence.
It is important to understand that chitta is not of this material realm, and is attached to neither the body nor the mind. It comes from the realm of the Divine and is the Divinity Within each of us. Chitta is not governed by death, transcends space and time and has the potential to transcend the karma of the body.
But while the person remains bewildered and ignorant of the true nature of the Self, the chitta will become saturated with the cares of the mind and be just as bewildered. Even after leaving the body, the chitta can carry traces of the old personality which can lead to tragic consequences.
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Chitta suffused with the physical mind
