- the universe is a physical one that is fundamentally made up of energy, matter and all that stuff. Every observer interacts with events within their event horizon
- the universe is a sequence of probability distributions over states at different times, and physics describe the state transition function. For every observer, few parts of the states distribution are collapsed because they are known, but events in the past, future or far away are unobserved and in the maximum entropy state. Reality is the set of possible states that have a positive probability for all observers
These models of the universe can be formulated such that they make exactly the same predictions about the world and are in some sense equivalent. Every statement that is formulated in the framework of 1) can be translated into an equivalent statement in the framework of 2), and therefore they are not really different theories about what the universe is.