Beyond or behind the gross physical reality are more "subtle" or "spiritual" dimensions of emotional, mental and spiritual consciousness and existence. These are collectively called the "subtle" worlds or body or consciousness, in contrast to the "gross" consciousness and body of external physical reality. They are not themselves the Absolute reality, but rather a sort of intermediate reality.
Yet apart from the above definition, which more or less all esoteric and spiritual traditions and teachings agree on, there is no common definition. Instead, "Subtle Being" or "Subtle Reality" is used in a rather arbitrary manner, with different definitions, sometimes related and sometimes distinct, adopted by different authors. Only a few are listed here.
In Vedanta (Mandukya Upanishad and later teachings - e.g. Advaita - based on it) "subtle" is used to designate the "dream state" of consciousness, and in Advaita this also includes the Prana, Manas, and Vijnana koshas (= the vehicles of vital force, mins, and higher consciousness) re-interpreted from of the Taittiriya Upanishad.
In Tibetan and Tantric Buddhism it refers to an intermediate grade between the "gross" and "very subtle" "minds" and "winds" (vayu = prana).
The Sukshma Sthula or Subtle Body is one of the seven principles of man in Blavatskian Theosophy; it is also called the "astral body" (this has little similarity with the astral body of Out of Body experience, because it cannot move far from the gross physical vehicle, it seems to correspond to what Robert Monroe calls the "second body" ( Far Journeys, p.78)), and identified with the Double or Ka
In Sant Mat / Radhasoami cosmology - the Anda (Cosmic Egg) / Sahans-dal Kanwal (Crown Chakra) is sometimes called the Subtle; hence Subtle = Astral
The term Subtle Physical is used somewhat generically by Sri Aurobindo (in Letters on Yoga) to refer to a wider reality behind the external physical.
Ken Wilber uses the term Subtle to indicate the yogic and mystic holonic-evolutionary level intermediate between "Psychic" (in his series = Nature Mysticism) and "Causal" (=Realisation"); it includes many psychic and occult experiences and can be considered as pertaining to the Subtle as defined here (although it also includes other realities and experiences that might also be interpreted as "Inner Gross" - e.g. Kundalini as a classic example).
The above list is in no way definitive - no doubt other descriptions in different esoteric traditions can be added here as well.
Although all the above realities each resenting experiences pertaining to their own specific spiritual and yogic tradition - are not necessarily synomous (In Sant Mat the Sahans-dal Kanwal is associated with the Astral, while Sri Aurobindo distinguished between the Subtle Physical, the Vital, and the Inner Being), in all cases they represent a larger reality behind and supporting the gross surface or outer being. While in Vedanta the emphasis is on yogic experience rather than metaohysics, and the definitions offered there are too broad or too vague (or if referring to the dream state alone, not applicable, as dreams pertain to the "gross" level) to be of much use in specific "type descriptions" of states of consciousness for an ontological map or cartography of states of existence. Tibetan Buddhist conceptions obviously refer to a series of ch'i levels, but these seem more related to the etheric or ch'i octaves of the inner or perhaps even the purushic being.
Generally, "Subtle" is the opposite of "gross", that is, of ordinary "gross" consciousness and mundane reality. "Inner" might be considered a synonym. Thus occultists might speak of the "inner planes" as opposed to ordinary external reality. "Inner" also emphasises the subjective or noetic nature of this reality.
A difference between them is that "inner" can include the inner or spiritual dimension of the material physical, whereas "subtle" is always supra-material in nature.
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