theory is viewed as primary in the AdS/CFT correspondence, our bulk spacetime might be considered emergent, but if the correspondence is a true duality, the bulk spacetime is present from the start. The hypothesis of spontaneous dimensional reduction at short distances [@Carlip2; @Carlip3], the dynamical generation of extra dimensions [@Arkani], and some “CFT primary” forms of the AdS/CFT correspondence [@Murugan] suggest another possibility: some dimensions may be emergent while others are not.

I am omitting a third category, models in which spacetime, the metric, and diffeomorphism invariance are present, but the *dynamics* of gravity is emergent. The archetype is Sakharov’s induced gravity [@Sakharov; @Adler], in which the Einstein-Hilbert action first appears as a counterterm in the matter action.[^1] From the point of view of effective field theory [@Burgess], the distinction between such models and general relativity is marginal: an effective action includes all possible terms, and it is not clear that one can distinguish their origins. Also in this category are models in which the dynamics of the metric is determined by thermodynamics [@Jacobson; @Padmanabhan; @Pad1], or as a consistency condition for the propagation of other fields [@Pad2]. While these are certainly interesting, they fail my (perhaps narrow) criteria for 