What we can instead ask is whether there could be a

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observe. What we can instead ask is whether there could be a higher-level description, emergent from our ontology, that can successfully account for our world. Such a description is not forced on us at the God’s-eye (or Laplace’s-Demon’s eye) view of the world. It would always be possible to say that reality is a vector in Hilbert space, evolving through time, and stop at that. But that’s not the only thing we’re allowed to say. The search for emergent levels is precisely the search for higher-level, non-fundamental descriptions that approximately capture some of the relevant dynamics, perhaps on the basis of incomplete information about the fundamental state. The question is whether we can recover the patterns and phenomena of our experience (space, objects, interactions) from the behavior of our fundamental ontology. To get our bearings, consider the classic case of N massive particles moving in three- dimensional space under the rules of classical Newtonian gravity. The state of the system is specified by one point in a 6 N -dimensional phase space. Yet there is an overwhelming temp- tation to say that the system “really lives” in three-dimensional space, not the 6 N -dimensional phase space. Can we account for where that temptation comes from without postulating any a priori metaphysical essence to three-dimensional space? There are two features of the description as N particles that make it seem more natural than that featuring a single point in phase space, even though they are mathematically equivalent. The first is that the internal dynamics of the system are more easily interpreted in the N - particle language. For example, it is immediately clear that two particles will strongly affect each other when they are nearby and the others are relatively far away. This kind of partial and approximate understanding of the dynamics is transparent in the N -particle description, 6
and obscured in the point-in-phase-space description. The second is that the system looks like N particles. That is, in the real-world analogues of this toy model, when we observe the system by interacting with it as a separate physical system ourselves, what we immediately see are N particles. There can be multiple equivalent ways of describing the internal dynamics of a system, but the one we think of as “natural” or describing what “really exists” is often predicated on how that system interacts with the outside world. Similar considerations apply to familiar examples of emergence, such as treating a box of many atoms as a fluid. In this case the two descriptions are not equivalent – the emergent fluid description is an approximation obtained by coarse-graining – but the same principles apply. The internal dynamics of the particles in the box are more easily apprehended in the fluid description (fewer variables and equations are required, given some short-distance coarse-graining scale), and we can measure the fluid properties directly (using thermometers and barometers and so on), while the states of each individual atom are inaccessible to us. Emergent structures that

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