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Relationship Theory
The metaphysical basis for Mythorealism is something called Relationship Theory, which provides us both with a definition of reality as we understand it, and a basis for making critical judgments about truth and falsehood. It also provides us with a method of thinking known as Ringing the Changes, which allows us to view the various perspectives on reality with mental fluidity.
Relationship Theory was discovered by David Douglas Thompson more than thirty years ago, and further developed over the years in conversation with friends and family members. Jason Thompson, in particular, was heavily involved in developing the finer points of the theory. More recently, C.S. Thompson has described Relationship Theory in detail in his novella The Forbidden Science.  The concepts involved in Relationship Theory are too complex to present in the limited space available here, but the core idea behind the theory is that reality is defined by relationships. A thing cannot be known in and of itself but only in relation to other things, thus there is no absolute or objective knowledge available to us. Relationships themselves are "absolute reality."

Objective reality is something opaque to us, and something that must remain forever opaque. We see the always-changing shadowplay of light and dark, the flickering images on the wall in front of us, but not the thing that casts the shadow. Not the thing in itself. The true nature of reality is beyond our senses. We can never touch it or measure it. We can never know it.
We have only points of view. Each point of view describes the world as best it can, but nowhere is it stated that all points of view are equally valid. Reality, after all, is something we interact with, something we have a relationship to. There are always observable facts, whatever they might mean.
It is not claimed that reality is what we believe it to be or wish it to be, only that it is what we experience. Another way of expressing the same thing is that reality is our relationship with observed phenomena.
If you are dealing with something the absolute truth of which is unknowable, then all explanations which account equally well for the knowable facts are equally valid from a logical standpoint. You must then apply a pragmatic truth-test, which is relative effectiveness. And that will vary to a large extent from one situation to another, from one person to another, and from one time to another as well. Thus the existence of different worldviews. Those worldviews which fail to account for the facts can be set aside. But there will remain a set of worldviews which do account for the knowable facts, and which seem to do so equally well. In this case you can only discriminate between them based on effectiveness, on their actual use in the lives of those who hold them. And that will not be an absolute truth but a relative one. (From The Forbidden Science by C.S. Thompson)

The core principles of Relationship Theory can be expressed as follows:

1
Reality is what we experience or perceive.
2
Perception occurs through the meeting of unlike things, or in other words, there is no perception without contrast.
3
The perception of contrast occurs through an interaction, a relationship between the parties involved.
4
For information to be perceived, three parties are required: an observer and two objects, which change relative to each other.
5
To perceive anything is to change it; therefore a thing cannot be known in and of itself, but only as part of a particular relationship.
6
All changes are balanced by equivalent but opposite changes, setting up the range of all possible future changes.
7
Because of the balance of changes between opposing points of view in a closed system, we know that all points of view are relative, and that none of them can be privileged as absolutely true. All accurate descriptions of a system are equally valid, so the truth-test of anything is the degree of its accuracy, and where that is unknowable, its effectiveness. A thing cannot be absolutely true or false, but only true or false in particular interactions, or possibly in the vast majority of interactions. We can say something is very true or somewhat true, but not absolutely true. Reality is a matter of shades of gray, of particular relationships.