Observers and the physical universe
Metaphysics has no need of the observer concept because, by definition, a metaphysical statement cannot be empirically validated. In contrast, physics needs observers because, by definition, a physical statement is one that can be empirically validated or disproved. According to Wheeler, only observers are significant in science:
Stronger than the anthropic principle is what I might call the participatory principle. According to it we could not even imagine a universe that did not somewhere and for some stretch of time contain observers because the very building materials of the universe are these acts of observer-participancy. You wouldn’t have the stuff out of which to build the universe otherwise. This participatory principle takes for its foundation the absolutely central point of the quantum: No elementary phenomenon is a phenomenon until it is an observed (or registered) phenomenon.
According to Wheeler’s participatory principle, the relationship between observers and physical reality is contextual, that is, it depends on the theory and apparatus involved. This is seen clearly in the differences between classical mechanics (CM) and quantum mechanics (QM) and is a principle we adopt throughout this book.