The premise I've been pursuing in this book is that the essence of existence starts with the Cartesian observation that "I think therefore I am." 

I and you my reader share this undeniable quality. It is so native, so obvious, so familiar a fact to every person that it tends to remain unremarked and unexamined. 

Descartes was not the first to make this observation. It clearly underlies all religious intuition, from the earliest evidence of human thought. Cave paintings preserved from antiquity convey this sense of self as the Thinker, the Observer. A newborn babe conveys the same thing clearly.

The next obvious observation is that the nascent Observer is not alone. There are Others. It becomes evident to each of us that these Others are Observers too. 

With that it soon becomes apparent that these Others are in a Place, a shared space, an environment. And almost immediately it becomes obvious that this environment is not unchanging. Details shift. Most obviously the locations of Others as they move around. But the locations of objects - blankets, chairs, books - may change, usually by the actions of Others.

All this seems too obvious to mention. Here is here. Home is home. Which is exactly why it is mentioned so rarely.

What these changing observations reveal is the nature of Now. Now is an instant of observation.

Notice that Now is not external. Now is in the mind of the Observer. It is not an artifact of external reality. It is an artifact of Thought. Of mind. Of spirit. It is an amalgam of inputs of reality that have been transformed into an an instant of Thought. One unique instant of thought, available to one unique Observer. Now is the instant when what was Possible for this Thinker, this Observer, became Probable, and then became Real.

In the next instant, what is Real becomes Memory and a new, slightly altered, Reality takes its place.

And this same process, different only in detail, occurs for every other Observer.

It is what the physicist Richard Feynman identified as the Path Integral. For each Observer, every potential observation sums to one observation. Probability wave interference cancels out all but the one true path. For that Observer. 

What you get is what you see. Each viewpoint is unique. Each Observer experiences but one true path. Each observer has a unique inner Reality.

What we think of as external reality, then, is the assumed sum of the inner realities of all Observers. 

Again the path integral applies. For everyone in this multiverse of unique Observers, everything all of them together come to know of the world sums to one apparent, ever changing, Now of Reality. And that is the thing we call the Physical Universe.  

[Incidentally, we may now wonder why the nature of time assumed here seems to be ignored when science thinkers consider the quantum realm, their fundamental realm of probability. Why does the condition of the cat hidden in a box raise so many questions? The cat has its own evolving Now, which defines its condition. The person who opens the box has another evolving Now in which the condition of the cat is revealed to that one Other. And so on down the chain of Nows when the condition of the cat is disclosed to other Observers. Each of these Nows is unique to one Observer. For each of them the difference between the probable and the actual condition of the cat (and everything else) eventually gets resolved in their own Now. The rest is History.]