Mental plane
The mental plane, or world of thought, in Hermeticism, Theosophical, Rosicrucian, Aurobindonian, and New Age thought refers to the macrocosmic or universal plane or reality that is made up purely of thought or mindstuff. In contrast to Western secular modernist and post-modern thought, in occult and esoteric cosmology, thoughts and consciousness are not just a byproduct of brain functioning, but have their own objective and universal reality quite independent of the physical. This reality itself constitutes only one gradation in a whole series of planes of existence (the total number of planes varies, although seven is a  common number in Theosophical formulations). In most such cosmologies and explanations of reality, the mental plane is located between, and hence is intermediate between, the astral plane below and the higher spiritual realms of existence above. Predecessors of the concept. In India in the seventh century b.c.e., the Taittiriya Upanishad referred to five levels of self, of which the middle one is the "self made of mind" ("manas"). Although the text is describing the nature of the individual rather than the cosmos as a whole, it established the concept of mind as only one of a series of ontological layers  of being. The Taittiriyan concept of the five selves would represent an important element of Vedantic ontology, for example the five koshas of Advaita Vedanta. Meanwhile in Greece, and coming from a philosophical-mystical rather than a yogic perspective, Plato spoke of archetypal forms or ideas as the original spiritual prototypes behind the physical world. These ideas were not equivalent to mind or thought as such. But they did eventually help inspire Middle Platonic (including Philo's) and Neoplatonic metaphysics in which the ideas exist in the mind of God or the Demiurge, or (according to Plotinus and hence Neoplatonism ) the Divine Mind or "Nous". In the metaphysics of Proclus, the Nous is only one level of hypostasis, with higher ones like Life, Being, and Unity above it. Theosophical and Hermetic interpretations. The esoteric conception of the mental plane had to wait till the occult revival of the late 19th century, with the development of modern Theosophical, Hermetic, and Kabbalistic ideas that were to serve as the foundation for the current New Age movement. H. P. Blavatsky taught a cosmology and ontology consisting of seven principles and seven planes. In her writings she aimed at showing how different spiritual systems  share a common source, and therefore refers to Vedantic, Buddhist, Samkhyan, Tantric, Neoplatonic, Ancient Egyptian, Kabbalistic, and Occult systems. She relates the mind with the principle of "Manas", and also talks about a kosmic Manasic plane. However, the latter represents a very sublime level of consciousness which can be experienced only by enlightened beings. In "The Kabbalah Unveiled", MacGregor Mathers divides the sephirot (apart from the lowest, "Malkhut", representing the physical world) of the Tree of Life into three triads: the Intellectual, Moral, and Astral triads. The Intellectual triad could be compared to the Neoplatonic ideas (Intellect = "Nous") and  this here represents the highest subdivision. Around the turn of the twentieth century, Max and Alma Théon were producing "The Tradition". This involved a Lurianic Kabbalistic-inspired cosmology in which the ineffable Godhead emanates a number of sublime worlds, the lowest of which is made up of seven or eight "States". The third or fourth lowest State is the Mental or Intellectual, which again corresponds to a very high, spiritual level of consciousness. The world of thought in the Western Wisdom Teachings. According to Max Heindel's Rosicrucian writings, immediately beyond the higher regions of the desire world — which exhibit  the marked peculiarity of blending form and sound — and before entering the world of thought (mental plane), there is the "Great Silence", where all the world seems to disappear and the spirit has the feeling of floating in an ocean of intense light: all is one eternal now. The world of thought is, according to this author, divided into two regions: the "Region of Concrete Thought" (inferior) related to the mind and the "Region of Abstract Thought" (superior) related to the third (lower) aspect of the threefold Ego, the "Human Spirit" aspect. His writings, called "Western Wisdom Teachings", describe that  the Memory of Nature may be read, in an entirely different manner covering the essence of a whole life, in the highest subdivision of the "Region of Concrete Thought" of the "World of Thought". A discipline prevalent in the West is mathematics. The field was strengthened by Pythagoreanism and Philosophy (Platonism, Neoplatonism), which were, alongside ancient Egyptian philosophy, among the oldest non-mythical wisdom paradigms. Math once meant "magic" and is still important within occultism, especially topics farther beyond the earthly. Any mathematician who considers the mental plane's dimensionality equal to or more than the earthly one defines them  as "hyperplanes." Classical Theosophists (other precursors of Heindel) may not have defined those dimensions, but several neo-Theosophists call the mind 5-dimensional, (i.e. in Einstein-Minkowski spacetime) Platonic terms such as nous or Nous (human or divine mind), synonymous (in contexts) with logistikon/logos or Protologos/Logos-Alogos, and Aristotelean (empiricist, including much/all constructivism) acceptance/use of the terms differ. They may reject that Logos Alogos causes Protologos or Logoi cause/are Nous, or that such or infinite dimensions enumerated in abstract philosophical logic have any use in semi-abstract/-concrete mathematical logic. However, contemporary philosophers are considering dimensions  of M-theory physics. Some current definitions of the mental plane. The influence of C.W. Leadbeater's work on the New Age movement has been underrated. One of Leadbeater's achievements was to make Blavatsky's difficult cosmology simpler and more understandable; for example by equating each of the seven principles or vehicles of consciousness with a corresponding cosmic plane. So there is a physical body that is the vehicle of consciousness on the dense matter of the physical plane; an etheric body, which is formed by the etheric matter found on subtler counterpart of the physical plane; an astral  body that corresponds to the astral plane; a mental body which belongs to the lower mental plane, and a causal body, or vehicle of the individual soul, which is located at the level of the higher mental or Causal plane. Thus, for Leadbeater and subsequent theosophists, ex-theosophists (e.g. Alice Bailey) and occultists, the mental plane is a distinct reality or zone of being, more subtle and refined than the Astral, but denser and coarser than the Causal. A detailed account of The Mental Plane and the Mental Body and their associated phenomena, as described by Leadbeater and his co -worker Annie Besant (who succeeded Blavatsky as head of the Theosophical Society), can be found in Arthur E. Powell's "The Mental Body". Sri Aurobindo developed a very different concept of the mental plane, through his own synthesis of Vedanta (including the Taittiriya Upanishad), Tantra, Theosophy, and Theon's ideas (which he received via The Mother, who was Théon's student in occultism for two years). In this cosmology, there are seven cosmic planes, three lower, corresponding to relative existence (the Physical, Vital, and Mental), and four higher, representing infinite divine reality ("Life Divine" bk. 1 ch. 27). The Aurobindonian  Mind or mental plane constitutes a large zone of being from the mental vital to the overmental divine region ("Letters on Yoga", Jyoti and Prem Sobel 1984), but as with the later Theosophical concept it constitutes an objective reality of pure mind or pure thought.