Universes are everything from a particular point of view.
This book describes several universes. Since the term universe is generally construed to be all-inclusive, it may seem counterintuitive to have more than one of them. On the one hand, the physical universe contains all of the other universes as parts. When referring to this all-inclusive universe, it is known as the universe. However, there are other entities which are in some sense unbounded, in light of which they will also be called universes. It is of course odd to have multiple unbounded entities, especially if some are parts of another, so the existence of multiple unbounded entities must be further elaborated.
To reiterate, the universe is that which contains absolutely everything: in this absolute sense, there can clearly be only one universe (which is why the definite article is emphasized in this context). However, the universe as seen from a particular reference point is also a universe: it is a universe from the subjective point of view (or point of reference). These subjective universes, based on particular points of view, are composed of references to the containing universe.
Subjective universes exist within the universe, as well as being universes in their own right. Just as a reference is itself a thing and a reference to a thing, so a universe which consists of references is both a referential universe and contained in the universe (to which its references refer). To use a more concrete example: spoken words may stand for something else, but they are also themselves sounds. So the universe of spoken words is both contained in the universe of sounds, and it is a universe of its own (when the words are understood as references). When a given referential universe is viewed in relationship to the universe, it is seen to be a part of it: in that larger context (or from that larger perspective), these referential universes are merely parts. On the other hand, when they are viewed from their own perspective, they operate as the entire universe, in the sense that nothing exists outside of them from that perspective .
To borrow an example from a later section of this book, the subjective universe is everything that an individual can perceive. Everything, from a subjective point of view, is the entire field of perception. Although the subjective universe can be restricted by attention, which limits what is perceived (or conceived), a given perceiver will never perceive outside of their perceptual universe. In this sense, it is complete: it is a whole, or a totality. From the subjective perspective, references to physical things are everything that exists: therefore, subjective experience forms a universe. Similarly, although concepts may be restricted to some domain of discourse , concepts also form a universe. The conceptual mind lives in a universe of concepts, in that nothing can be conceived which is not a concept.
To use a more concrete example, our house may be a part of the world which we enter and leave, but if we never leave it, it is our universe. People may come and visit, and tell us of the world outside, and we may form an idea of the world outside, but we still form an idea of the world outside from within our house . There is nothing inside of our house which is outside of our house. Universes are like this; it is possible to have references to things outside of a universe from inside of a universe, and many things can be accommodated (referentially) within that universe. From the referential perspective, the set of references is complete, unbounded, and whole. However, from the perspective of the larger container, references are categorically different than the things they point to, and these universes are incomplete, bounded, and merely parts.
To return to the subject of the (physical) universe, it is defined to contain all of the other universes. This containment relationship between the physical and other universes is often taken for granted, but it is not the only possibility. For example, we might believe that the subjective universe contains the physical universe. In other words, our only knowledge of the external world comes through experience with the subjective world, so it is not possible to confirm that there is an objective world independent of the subjective world.[8]
[8] The discussion of the way that different universes relate to one another is analogous to the philosophical debate about monism and dualism. This discussion focuses on the relationship between the world of ideas (or perhaps the world of the spirit) and the world of matter. Roughly, dualists believe that both matter and spirit exist, and that they are different; monists believe in the ultimate existence of either only ideas or only matter (these two subgroups are called idealists and materialists, respectively).