freedom are merely asymptotic values of ordinary bulk degrees of freedom, but I do not know how to make this distinction precise.

Classifying emergent models \[cats\]
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My aim is to describe obstacles facing all models of emergent gravity, rather than focusing on details of any particular approach. Still, it will be useful to distinguish two broad categories of models that face rather different challenges.

The first category, which I will call type I, comprises models in which the fundamental degrees of freedom live in some sort of “environment”: a medium, a lattice, a pre-existing space or spacetime, or the like. Examples include analog models based on fluid flows and similar phenomena [@LV; @Unruhb; @Wein; @Hu], gravity-like excitations near the Fermi point [@Volovik; @Volovik2], quantum Hall effect edge states [@Zhang], deformations in an elastic solid [@Zaanen], and spin systems on a fixed lattice [@Wen; @RD]. The background environment can have fairly minimal structure; in “pregeometric” models, for example, composite gravitons appear in what is initially a purely topological manifold [@Akama; @Amati; @Floreanini; @Wetterich; @Alfaro]. Type I models must typically decouple the environment from observable quantities quite strongly to reproduce observation.

The second category, which I will call type II, comprises 