The mental plane, as its name implies, is that which belongs to consciousness working as thought; not of the mind as it works through the brain, but as it works in its own world, unencumbered with physical spirit-matter. This world is the world of the real man.
The word ‘‘ man ” comes from the Sanskrit root “ man,” and this is the root of the Sanskrit verb “to think,” so that man means thinker; he is named by his most characteristic attribute, intelligence. In English the word “mind” has to stand for the intellectual consciousness itself, and also for the effects produced on the physical brain by the vibrations of that consciousness ; but we have now to conceive of the intellectual consciousness as an entity, an individual—a being, the vibrations of whose life are thoughts, thoughts which are images, not words.
This individual is Manas or the Thinker, he is the Self clothed in the matter, and working within the conditions, of the higher subdivisions of the mental plane. He reveals his presence on the physical plane by the vibrations he sets up in the brain and nervous system; these respond to the thrills of his life by sympathetic vibrations, but in consequence of the coarseness of their materials they can only reproduce a small section of his vibrations, and even that very imperfectly.
Just as science asserts the existence of a vast series of etheric vibrations, of which the eye can only respond to a small fragment, the solar light-spectrum, because it can only vibrate within certain limits; so can the physical thought-apparatus, the brain and nervous system, only think a small fragment of the vast series of mental vibrations set up by the Thinker in his own world.
The very receptive brains respond up to the point of what we call great intellectual power; the exceptionally receptive brains respond up to the point of what we call genius; the exceptionally unreceptive brains only respond up to the point we call idiocy; but everyone sends beating against his brain millions of thought-waves to which it cannot respond, owing to the density of its materials, and just in proportion to its sensitiveness are the so-called mental powers of each. But before studying the Thinker, it will be well to consider his world, the mental plane itself.
The mental plane is that which is next to the astral, and is separated from it only by differences of materials, just as the astral is separated from the physical. In fact, we may repeat what was said as to the astral and the physical with regard to the mental and the astral.
***From Annie Besant – The Ancient Wisdom