Within the various forms of Buddhist meditation, after one has learned to settle the mind - samatha - there comes a time where true insight and the ability to see things clearly as they are - vipassanā - is reached. In the method that leads to realizing the Dhammakāya, as was taught in the Wat Pak Nam temple in Bangkok, Thailand by the famous meditation master Luang Pu Sodt Jantasaro, there exists not just one inner - or what we westerners refer to as 'astral' - body, but multiple. He made reference to succeedingly refined bodies and minds linked within our coarse corporeal body.
The corporeal body that we currently use is composed of the four major elements dhātu, within the element of space ākāsa-dhātu, compiled from and for existence on this place we call Earth. Then there is the mind element viññāṇa-dhātu. These elements are linked together, constituting our body and mind. If they were not linked together we have either a corpse or a spirit without a body. Using this reasoning, mind and body account for the fundamental pillars for living a trended existence on this planet.
In the various Pāli and Sanskrit sūtras, Lord Gautama Buddha discussed the existence of other worlds and life forms, categorizing them into three major realms of: กามภพ kāmma-loka, รูปภพ rūpa-loka, อรูปภพ arūpa-loka. Each of these major realms have varying divisions and levels. Then there is Nirvāṇa, outside the cycle, yet still in the matrix. Not a great fan of Wiki for sūtra research, but for now:
For each of these realms, heavens, or abodes (whatever you want to call them) there are said to be beings that reside in them. A realm then is a place meant to be a container for the beings that reside there to have an existence within it. If we follow this logic - or what some call "facts of the universe" - and the principle that the coarse body we use is composed of and for this earth, then that would naturally mean that for each other realm there would be a corresponding body / form kāya designed for that place. Made to be housed in that place, composed of its elements, coarse and or refined.
The theory (and dare I say, truth) is that all these bodies exist simultaneously in the coarse human body, while the mind is still connected to it. They overlap and are interlinked "ซ้อนกันอยู่". The linking, and - most notably - access point or ‘metaphysical umbilical cord’, just so happens to reside in the abdomen, two of one's own finger-widths above the level of the navel at what was appropriately referred to by Luang Por Sodt Jantasaro as the “center” ศูนย์กลางกาย.
This is a very very interesting word choice. The word ศูนย์ 'sūñña' comes from the Pāli word suññatā which means "emptiness", "zero-ing point", "point of origin". ศูนย์กลาง, pronounced 'soon-glaang' in Thai, refers to the geometric center of an object or shape. กลางกาย, pronounced 'glaang-gaai', means 'middle of the body'. กาย, pronounced 'gaai', comes from the Pāli word kāya, meaning "body" or "form".
When one starts to get to the dhammakāya-gotrabhū, it could be said that they are getting to the Buddha element, buddha-dhātu, the tathagatagharba. This is where a more reliable refuge can now be established because it is outside the three realms, is more refine and with high-levels of purity and power. However, one doesn’t really start reaching an accurate and true internal refuge until they reach the dharmakāya-arahanta, as this is the body/form/shelter kāya that is freed and pure. Also, at this level it is outside of the three characteristics of existence: atta ‘Self’ (though the experience of it is should not be compared to, nor put with the conations of ego, “I” or “mine”), sukha ‘true happiness’, and nijja ‘permanence’. This is the practical reasoning and logic to why it can be called a refuge. It is also said that there are even more refined and more powerful dharmakāyas, on and on, layered deeper within the dharmakāya-arahanta. I think this is an important point to note, especially when considering ‘atta’.
Luang Pu Sodt Jantasaro and Luang Por Dhammajayo state that once one can unify their coarse human body with their uber-refined dharmakāya-arahanta (or beyond that) then abhiñña immediately and automatically arises. To common sense, this would make sense because when you unify a mold or casing that is charged by the element/energy of Nirvāṇa, I would imagine some pretty amazing things happen.
However, I am also told that when one reaches the dharmakāya-gotrabhū then one’s systematic thinking processes and mental software programming changes. One no longer is hindered by the coarse and conditioned physical, mental, and emotional processing of samsara and kilesa. In this way, it is not possible and even futile to analytically attempt to conceive or understand the uber higher-caliber mode of thought of the dharmakāya-gotrabhū, never mind beyond. If one uses an instrument that is not up to par and incompatible for the task, then one cannot achieve the appropriate results.
Also, only when one gets to the gotrabhū state, and begin to be one with it, can he/she begin to practice vipassanā. Only then can that person truly begin to understand the Four Noble Truths. Another perspective is to look at the first of the four foundations of mindfulness in the satipatthana sūtra: kāyanupassi, (kāyanupassana; kāyanupassana-satipatti) “thus s/he dwells observing body in body internally”. Yes it could mean have mindfulness and awareness of the three characteristics of existence in the coarse human body. However, it could also just as equally be interpreted to mean ‘mindfully observing the existence of bodies within bodies inside the coarse human body’. Same goes for vēdananupassi, vēdananupassana. Seeing ‘feeling/emotional state/vēdana’ in vēdana. Each body and mind has an อารมณ์, a feeling, a state of being, a vēdana. And cittanupassi, cittanupassana: seeing mind in mind in mind, etc.
If we go into subjects such as vipassanā (vi: super, supramundane, ‘special’; passana: sight, the faculty of vision, seeing) or even cakkhu, like dibbacakkhu, dhamma-cakkhu, buddha-cakkhu, then there is mention of an eye that is doing the seeing. Can the eye refer to mind? Or to an eye/faculty of vision, which would necessitate a body/form kāya that holds and encases that eye/faculty of vision?
It is said that only when one reaches their dibbacakkhu, being before the other more refined cakkhu, when they can use it to see and know (ñāna-dassana) the other realms and worlds, and see the arising and passing away of beings within the universe and world systems. At this point, one can then witness beings going to different realms according to their previous mental, verbal, and bodily actions (kamma). This is how the Lord Gautama Buddha re-discovered the already existing Law of Karma, the cycle of birth and death, using his various cakkhu.
What about the reference to the element, or actual existence and abode of Nirvāṇa nibbāna-dhātu or āyatana-nibbāna that is in the sūtras? That is a whole 'nother subject. Buddhism is not annihilation-ism.
To look at this a different way, many people have experienced astral traveling. When journeying, many describe the existence of a silky - often described as liquid-silver like - cord, connecting the astral body to the coarse body. Why is the cord there? What is the cord’s purpose?
The refined human body in this 18 body explanation is the astral body, which is why it looks similar to the human body, yet is separate and still somehow connected to the coarse human body. This phenomenon, and such like it, is very well documented in astral traveling circles. Feel free to find a qualified teacher and go see for yourself if it is true!
I will defer to the quality of the Dharma as 'ehipassiko', inviting the partitioner to come, see, and experience it for themselves so they can then decide how they think and feel about it. This is the best way to conclude these discussion points. My intention here is to at least try to put forth effort into explaining the philosophical, practical, and historical significance here. As Wat Phra Dhammakaya is pretty strong in Thailand now, I thought readers and meditation practitioners may want to know more about the deeper aspects of the teachings of BuddhaDharma.
If there is any error or misinterpretation herein, I offer my apologies and state that it does not reflect upon my teachers, but on my own current short-comings.
In service. With metta, openness, and respect for the Dharma.