The main underlying theme of the book was the idea of “model-dependent realism,” the idea that your reality depends on the model you employ. I think this is an excellent point.
The core idea of “model-dependent realism” can be summarized by: “We build mental pictures, or models, of how the world works, and we interpret the world through those models. We don’t have direct access to the world as it actually is, but only to the world as it is filtered through our model.”
The billiard ball model of molecules, the double helix model of DNA, and Newton’s laws as a few numbers instead of every interacting atom in the object… these are all excellent models, that help us understand the world— often so effective and powerful in prediction, and give us strong impressions that there are some correct models and that the complex happenings around us could be reduced to simpler principles.
However, a model represents the world, but only the parts you need or only those you intend to look at now. By choosing some model, you can look at a piece of a far more complicated world, that helps you understand it, which is great, but this might be good only “locally.”
For the “grand picture” of the world, the book suggests a theory called M-theory, the network of theories (the networks of models). Each theory (each model) in the M-theory network is good at describing phenomena within a certain range. In other words, reality (or realities) lives (or live) in the network of multiple models. Each model can describe and explain certain properties.
I think the Universe is out there, but at the same time it is within us, because it depends on how we look at it and how we model it. For example, the wave/particle duality, where two very different theories accurately describe the same phenomenon. I agree with Hawking and Mlodinow that reality should be interpreted based upon these models, where “several models overlap in describing a particular subject, multiple, equally valid, realities exist.” Or perhaps, the Universe depends on how we look at it and how we model it, because we are already part of it, and there exists rather a very strong interaction between a part and the whole :)