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All the substance that exists in the meta-universe of existences, sometimes called the Metaverse, originates from two fundamental planes defining the nature of substance, the Negative and Positive material planes. Between these two planes there exists a balance, but one that is fluid and ever changing, animated in a timeless void. In effect, the two planes churn together, becoming conjoined in such a manner that time is transiently brought into existence . . . creating a time 'period' which lasts many billions of years, in which all other existence is birthed and ultimately destroyed again.
The churning, conjoined happenstance manifests, initially, as the Elemental Planes, which may occur in varying degrees depending upon the incident of consolidation. The present relationship, where the elements are balanced and made manifest in the manner we recognize, is mere chance; another moment of churn would create utterly different, unfamiliar balances - but these are not matters with which we ever need concern ourselves.
We describe the focus where the elemental planes form the reality we recognize as the Prime Material Plane. This plane comprises the familiar structure of 'reality,' or the known universe, which includes Earth, the Solar System, the unnumbered galaxies and the widest reaches of intergalactic space. This existence is often simply referred to as the 'Prime Material.' The name itself is a misnomer, incorrectly given due to the subjective position of those prejudiciously attached to their plane of occupation.
Coextensive with the Prime Material is the Prime Immaterial, more familiarly called the Ethereal plane, an insubstantial reflection of the more substantial world, occupied by ethereal shades whose existence tends to slip in and out of the Prime Material, randomly or upon the will of the ethereal creature. On Earth, ethereal creatures are categorized as manifestations.
Through the accumulation of thought, following a long evolution initiated by the present incident of consolidation, further planes of semi-existence (generally referred to as the Outer Planes) came into being. The cognitive fury of millions of the Prime Material's inhabitants steadily served, over the space of tens of thousands of years, to vivify - bring to life - the unmistakable reality of these Planes beyond the simple understanding of the universe's first inhabitants. As each planet in the Prime Material became home to intelligent life, each outer plane that was conceived and therefore made substance grew in service with the individual culture's imagination. Thus, our outer planes are occupied by our imaginations alone - the outer planes created by the thoughts of other planets are unique to those civilizations. Though infinite, they are outside our ability to reckon and therefore outside our ability to visit. We may only visit those planes that our reckoning could create.
The first outer plane, slowly appreciated in Paleolithic times, with belief but without articulation, emerged as a vast, empty void that we call the Astral Plane, where endless vortices spiral out now to the Higher Outer Planes, which were necessarily forced to attach themselves to the framework that was previously created. Thus, to travel to any other outer plane, we must first cross the Astral Plane - which may be thought of as the imaginative existence created by the animal brain of intelligence. All other planes whose names are familiar to us are truly 'outer' in the sense that they are beyond the Astral.
See Outer Planes