I actually have read some David Lewis — and I recall his famous and controversial stance that all possible worlds are actual worlds. I was never satisfied by this view as it doesn’t appear to leave many open problems — especially if, as you say, these worlds are isolated from each other. Tegmark seems to at least put forward a more fleshed out framework of how these worlds exist yet remain independent. Interesting take on his book and I really appreciate your insightful comment! I enjoy your comment at the end about your Internal Reality being a minimal definition of ‘you’.