_Manas_, or the Thinker, is the reïncarnating 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_.
In _Manas_ the thoughts of all lives are stored. That is to say: in any one life, the sum total of thoughts underlying all the acts of the life-time will be of one character in general, but may be placed in one or more classes. That is, the business man of to-day is a single type; his entire life thoughts represent but one single thread of thought. The artist is another. The man who has engaged in business, but also thought much upon fame and power which he never attained, is still another. The great mass of self-sacrificing, courageous, and strong poor people who have but little time to think, constitute another distinct class. In all these the total quantity of life thoughts makes up the stream or thread of a life’s meditation—“that upon which the heart was set”—and is stored in _Manas_, to be brought out again at any time in whatever life the brain and bodily environments are similar to those used in engendering that class of thoughts.
It is _Manas_ which sees the objects presented to it by the bodily organs and the actual organs within. When the open eye receives a picture on the retina, the whole scene is turned into vibrations in the optic nerves which disappear into the brain, where _Manas_ is enabled to perceive them as idea. And so with every other organ or sense. If the connection between _Manas_ and the brain be broken, intelligence will not be manifested unless _Manas_ has by training found out how to project the astral body from the physical and thereby keep up communication with fellowmen. That the organs and senses do not cognize objects, hypnotism, mesmerism, and spiritualism have now proved. For, as we see in mesmeric and hypnotic experiments, the object seen or felt, and from which all the effects of solid objects may be sensed, is often only an idea existing in the operator’s brain. In the same way _Manas_, using the astral body, has only to impress an idea upon the other person to make the latter see the idea and translate it into a visible body from which the usual effects of density and weight seem to follow. And in hypnotism there are many experiments, all of which go to show that so called matter is not _per se_ solid or dense; that sight does not always depend on the eye and rays of light proceeding from an object; that the intangible for one normal brain and organs may be perfectly tangible for another; and that physical effects in the body may be produced from an idea solely. The well-known experiments of producing a blister by a simple piece of paper, or preventing a real blistering plaster from making a blister, by force of the idea conveyed to a subject, either that there was to be or not to be a blister, conclusively prove the power of effecting an impulse on matter by the use of that which is called _Manas_. But all these phenomena are the exhibition of the powers of lower _Manas_ acting in the astral Body and the fourth principle—Desire, using the physical body as the field for the exhibition of the forces.