In order to comprehend clearly the supra-mundane plane, we have to know first about its opposite, the mundane plane (Lokiyabhumi). The mundane plane comprises those levels at which the things of the world have control over the mind. Very briefly, three levels are recognized in the mundane plane, namely: the sensual level (Kamavacara-bhumi), or the level of a mind still content with pleasures of every kind; the level of forms (Rupavacara-bhumi), the condition of a mind uninterested in sensual objects, but finding satisfaction in the various stages of concentration on forms as objects; and lastly the formless level (Arupavacara-bhumi), the yet higher level of a mind finding satisfaction in the bliss and peace of concentration on objects other than forms. These three levels in the worldly plane are the mental levels of beings in general. Regardless of whether we presume to call them human beings, celestial beings, gods, beasts, or denizens of hell, they are all included within the three worldly levels. The mind of a world-ling can at any particular time exist in any one of these three. It is not impossible. It is quite normal. As a rule, though, it will tend to fall back naturally to the unconsecrated sensual level; the human mind normally falls under the influence of the delightful in colors and shapes, sounds, odors, tastes and tactile objects. Only on certain occasions is it able to escape from the influence of these seductive things and experience the tranquility and bliss which comes from practicing concentration on forms or other objects. It all depends on concentration. At certain times, then, a person's mind may be located in any of these levels of concentration. In India at the time of the Buddha this must have been fairly common, because people who had gone in search of the tranquility and bliss associated with the various levels of concentration were to be found living in forests all over the country. At the present time such people are few, but it is nevertheless possible for the ordinary man to attain these levels. If someone in this world is in the process of experiencing the bliss of full concentration on a form, then for him "the world" consists of just that form, because he is aware of nothing else. At that time and for that person, "the world" is equivalent to just that one form, and it remains so until such time as his mental condition changes.
Even though a person dwelling in any of these three levels may have gained such bliss and calm tranquility that he has come to resemble a rock, a lump of earth, or a log of wood, yet grasping and clinging to self hood are still present. Also present are various kinds of desire, albeit of the finest and most tenuous sort, such as dissatisfaction with the state in which he finds himself, which prompts him to go in search of a new state. That desire for change constitutes karma, so such a person has not yet transcended the worldly state. He is not yet in the supra-mundane plane. A mind dwelling in the supramundane plane has transcended the world. It views the worldly state as devoid of essence, self, or substance, and will have nothing of it.
"Supra mundane" means "above the world," and refers to the mind, not the body. The body can be anywhere at all as long as living conditions are adequate. "Supra-mundane" simply describes a mind dwelling above the world.
The criteria for recognizing these four levels in the supra mundane plane are the various mental impurities which are in the course of being eliminated. The Buddha divided the impurities in this group into ten kinds. He called them the Fetters (Samyojana). These ten fetters bind man and all beings to the world, keeping people in the mundane plane. If a person starts to cut through these fetters and break loose, his mind gradually and progressively becomes freed from the worldly condition; and when he manages to cut through them completely, his mind becomes completely free, transcends the world for good and comes to dwell permanently in the supra mundane plane.
In reality things are impermanent, unsatisfactory and not selves; but all creatures are attracted by things and become attached to them simply through misunderstanding. The Buddhist practice, based on Morality (Sila), Concentration (Samadhi), and Insight (Panna), is a tool to be used for completely cutting out grasping and clinging. The objects of our clinging are the five aggregates: body, feeling, perception, active thinking and consciousness. When we have come to know the true nature of the five aggregates, we understand all things so well that desire gives way to disenchantment, and we no longer cling to any of them.
What we have to do is lead the kind of life described as Right Living (Samma Vihareyyum), and be full day and night with the joy that arises out of conduct that is consistently good, beautiful and right. This limits aimless wandering of the thoughts and makes it possible to concentrate and to have clear insight at all times. Then if conditions are right, the result is disenchantment, struggle to break loose, slipping free, or even complete Nirvana.