(HPB, Articles, (Psychic and Noetic Action,) Part I)
........no physiologist, not even the cleverest, will ever be able to solve the mystery of the human mind, in its highest spiritual manifestation, or in its dual aspect of the psychic and the no'tic or the manasic), nor even to comprehend the intricacies of the former on the purely material plane--unless he knows something of, and is prepared to admit the presence of this dual element. This means that he would have to admit a lower (animal), and a higher (or divine) mind in man, or what is known in Occultism as the "personal" and the "impersonal" Egos. For, between the psychic and the no'tic, between the personality and the individuality, there exists the same abyss as between a "Jack the Ripper," and a holy Buddha. Unless the physiologist accepts all this, we say, he will ever be led into a quagmire. We intend to prove it.
(HPB, Articles, (Psychic And Noetic Action,) Part II)
Since the metaphysics of Occult physiology and psychology postulate within mortal man an immortal entity, "divine Mind," or Nous, whose pale and too often distorted reflection is that which we call "Mind" and intellect in men--virtually an entity apart from the former during the period of every incarnation--we say that the two sources of "memory" are in these two "principles." These two we distinguish as the Higher Manas (Mind or Ego), and the Kama-Manas, i.e., the rational, but earthly or physical intellect of man, in cased in, and bound by, matter, therefore subject to the influence of the latter: the all-conscious SELF, that which reincarnates periodically--verily the WORD made flesh!--and which is always the same, while its reflected "Double," changing with every new incarnation and personality, is, therefore, conscious but for a life-period. The latter "principle" is the Lower Self, or that, which manifesting through our organic system, acting on this plane of illusion, imagines itself the Ego Sum, and thus falls into what Buddhist philosophy brands as the "heresy of separateness." The former, we term INDIVIDUALITY, the latter Personality. From the first proceeds all the noetic element, from the second, the psychic, i.e., "terrestrial wisdom" at best, as it is influenced by all the chaotic stimuli of the human or rather animal passions of the living body. The "Higher Ego" cannot act directly on the body, as its consciousness belongs to quite another plane and planes of ideation: the "lower" Self does: and its action and behavior depend on its free will and choice as to whether it will gravitate more towards its parent ("the Father in Heaven") or the "animal" which it informs, the man of flesh. The "Higher Ego," as part of the essence of the UNIVERSAL MIND, is unconditionally omniscient on its own plane, and only potentially so in our terrestrial sphere, as it has to act solely through its alter ego--the Personal Self. Now, although the former is the vehicle of all knowledge of the past, the present, and the future, and although it is from this fountain-head that its "double" catches occasional glimpses of that which is beyond the senses of man, and transmits them to certain brain cells (unknown to science in their functions), thus making of man a Seer, a soothsayer, and a prophet; yet the memory of bygone events--especially of the earth earthy--has its seat in the Personal Ego alone. No memory of a purely daily-life function, of a physical, egotistical, or of a lower mental nature--such as, e.g., eating and drinking, enjoying personal sensual pleasures, transacting business to the detriment of one's neighbor, etc., etc., has ought to do with the "Higher" Mind or Ego. Nor has it any direct dealings on this physical plane with either our brain or our heart--for these two are the organs of a power higher than the Personality.
(Wm. Q. Judge, The Ocean Of Theosophy, Ch VII.)
Manas, or the Thinker, is the reincarnating being, the immortal who carries the results and values of all the different lives lived on earth or elsewhere. Its nature becomes dual as soon as it is attached to a body. For the human brain is a superior organism and Manas uses it to reason from premises to conclusions. This also differentiates man from animal, for the animal acts from automatic and so-called instinctual impulses, whereas the man can use reason. This is the lower aspect of the Thinker or Manas, and not, as some have supposed, the highest and best gift belonging to man. Its other, and in theosophy higher, aspect is the intuitional, which knows, and does not depend on reason. The lower, and purely intellectual, is nearest to the principle of Desire, and is thus distinguished from its other side which has affinity for the spiritual principles above. If the Thinker, then, becomes wholly intellectual, the entire nature begins to tend downward; for intellect alone is cold, heartless, selfish, because it is not lighted up by the two other principles of Buddhi and Atma.
(HPB, Key To Theosophy, Section X, pgs. 178-9)
The clue lies in the double consciousness of our mind, and also, in the dual nature of the mental "principle." There is a spiritual consciousness, the Manasic mind illumined by the light of Buddhi, that which subjectively perceives abstractions; and the sentient consciousness (the lower Manasic light), inseparable from our physical brain and senses. This latter consciousness is held in subjection by the brain and physical senses, and, being in its turn equally dependent on them, must of course fade out and finally die with the disappearance of the brain and physical senses. It is only the former kind of consciousness, whose root lies in eternity, which survives and lives for ever, and may, therefore, be regarded as immortal. Everything else belongs to passing illusions.
Try to imagine a "Spirit," a celestial Being, whether we call it by one name or another, divine in its essential nature, yet not pure enough to be one with the ALL, and having, in order to achieve this, to so purify its nature as to finally gain that goal. It can do so only by passing individually and personally, i. e., spiritually and physically, through every experience and feeling that exists in the manifold or differentiated Universe. It has, therefore, after having gained such experience in the lower kingdoms, and having ascended higher and still higher with every rung on the ladder of being, to pass through every experience on the human planes. In its very essence it is THOUGHT, and is, therefore, called in its plurality Manasa putra, "the Sons of the (Universal) mind." This individualized "Thought" is what we Theosophists call the real EGO, the thinking Entity imprisoned in a case of flesh and bones. This is surely a Spiritual Entity, not Matter, and such Entities are the incarnating EGOS that inform the bundle of animal matter called mankind, and whose names are Manasa or "Minds." But once imprisoned, or incarnate, their essence becomes dual: that is to say, the rays of the eternal divine Mind, considered as individual entities, assume a two-fold attribute which is (a) their essential inherent characteristic, heaven-aspiring mind (higher Manas), and (b) the human quality of thinking, or animal cogitation, rationalized owing to the superiority of the human brain, the Kama-tending or lower Manas. One gravitates toward Buddhi, the other, tending downward, to the seat of passions and animal desires.
(Further reading references)
(Wm. Q. Judge, The Ocean Of Theosophy, ch. VII)