Plane theory is a field of study pertaining to the nature of the Plane or Planes of Existenca. It attempts to explain the nature of the various Planes that appear to exist, as well as the relationships between these.
Plane theory and Magic
In academic discussion, Plane theory almost always refers to discussions on the nature of Magic -- specifically, whether Magic originates in the Borost:Material Plane, and, if it does not, which specific or theorized plane gives rise to Magic.
This main lines of inquiry primarily attempt to answer the questions: Which plane (or planes) is the source of Magic? Can this source be grasped or comprehended by an observer?
Single plane theory
Magical single plane theory postulates that Magic arises from the Material Plane itself. Perhaps its most important conclusion, the premises of Single plane theory, if accepted, postulate that an observer can, under appropriate conditions, perceive and interact with the entirety of the Material plane. This presents the hopeful idea that, at some point in the future, scholars will be able to obtain knowledge of the true nature of Magic.
Compound plane theory
Magical compound plane theory is a faction of Single plane theory that postulates that, although Magic originates from the Borost:Material Plane, the Material Plane is subdivided into a compound number of perceptions and realities, which may be unavailable to an observer, even with total freedom of conditions. These are known as Demiplanes.
Compound plane theory theoretically helps to explain why true knowledge of the nature and characteristics of Magic appears to be inaccessible to the denizens of the Material Plane.
Multiple plane theory
Magical multiple plane theory postulates that Magic arises from a wholly different plane of existence than the Material Plane. It was developed recently by Moras Gleckstett, and is currently the norm among academia.
Most commonly, adherents of Multiple plane theory cite the Ethereal Plane as the primary origin of magic and magical effects.
Multiple plane theory and Compound plane theory are often confused for each other. Compound plane theory argues that Magic arises from a Demiplane of the Borost:Material Plane; meanwhile, Multiple plane theory asserts that Magic arises from a wholly different plane.
Dimensional plane theory
Magical dimensional plane theory postulates that Magic arises from a number of planes tangential to the Dimensions that an observer is able to perceive. For this reason, the rise of Magic from a set of planes is Relative, which its adherents suggest is why some spellcasters can be so much more powerful than others.
Dimensional plane theory is also known as Relative plane theory.
A being of extraordinary intelligence and insight, for example, will be able to perceive and interact with more Planes of Existence than a knave or a fool, for which reason she will be able to generate more powerful magical effects.
Multi-dimensional plane theory
Multi-dimensional plane theory specifically states that any observer must be able to perceive at least four different dimensions (generally, three spatial dimensions and a temporal dimension) if it is to be regarded as a valid observer (i.e. spellcaster) in Dimensional plane theory. This is known as the Fundamental Theorem of Multi-dimensional plane theory, and its ramifications are extensive, as it allows a far more concrete understanding of Dimensional plane theory. Nonetheless, this Theorem is attacked by many adherents of Dimensional plane theory.
Number of planes
The number of planes is one of the points frequently discussed by Plane Theory. This line of inquiry attempts to answer the question: How many planes of existence are there?
The Borost:Material Plane is almost universally accepted to exist, with a few controversial adversaries.
The Ethereal Plane is also almost universally recognized to exist, and is theorized to be superimposed over the Material plane.
The fundamental difference between the Ethereal Plane and the Material Plane is that an observer in the Ethereal Plane could perceive and interact with beings and objects in the Material Plane, and even the Material Plane in its entirety, while an observer in the Material Plane can only rarely perceive -- let alone interact with -- beings and objects in the Ethereal Plane.
The Borost:Spiritual Plane is a theorized plane of existence, the residence of deities and deific beings.
The Abyssal plane, more commonly known as the Abyss, is a theorized plane of existence devoid of any matter.