If you can grasp that, then it ought not to be difficult to see further that it [consciousness] can subjectively formulate itself as a physical, a vital, a mental, a psychic consciousness -all these are present in man, but as they are all mixed up together in the external consciousness with their real status behind in the inner being, one can only become fully aware of them by releasing the original limiting stress of the consciousness which makes us live in our external being and become awake and centred within in the inner being. As the consciousness in us, by its external concentration or stress, has to put all these things behind -behind a wall or veil, it has to break down the wall or veil and get back in its stress into these inner parts of existence -that is what we call living within; then our external being seems to us something small and superficial, we are or can become aware of the large and rich and inexhaustible kingdom within. So also consciousness in us has drawn a lid or covering or whatever one likes to call it between the lower planes of mind, life, body supported by the psychic and the higher planes which contain the spiritual kingdoms where the self is always free and limitless, and it can break or open the lid or covering and ascend there and become the Self free and wide and luminous or else bring down the influence, reflection, finally even the presence and power of the higher consciousness into the lower nature.
     Now that is what consciousness is -it is not composed of parts, it is fundamental to being and itself formuates any parts it chooses to manifest -developing them from above downward by a progressive coming down from spiritual levels towards involution in Matter or formulating them in an upward working in the front by what we call evolution. If it chooses to work in you through the sense of ego, you think that it is the clear-cut individual "I" that does everything -if it begins to release itself from that limited working, you begin to expand your sense of "I" till it bursts into infinity and no longer exists or you shed it and flower into spiritual wideness.
It all depends upon where the consciousness places itself and concentrates itself. If the consciousness places or concentrates itself within the ego, you are identified with the ego -if in the mind, it is identified with the mind and its activities and so on. If the consciousness puts its stress outside, it is said to live in the external being and becomes oblivious of its inner mind and vital and inmost psychic; if it goes inside, puts its centralising stress there, then it knows itself as the inner being or, still deeper, as the psychic being; if it ascends out of the body to the planes where self is naturally conscious of its wideness and freedom it knows itself as the Self and not the mind, life or body. It is this stress of consciousness that makes all the difference. That is why one has to concentrate the consciousness in heart or mind in order to go within or go above. It is the disposition of the consciousness that determines everything, makes one predominantly mental, vital, physical or psychic, bound or free, separate in the Purusha or ivolved in the Prakriti.