  We construct an analytic theory of many-body localization (MBL) in random
spin chains. The approach is based on a first quantized perspective in which
MBL is understood as a localization phenomenon on the high dimensional lattice
defined by the discrete Hilbert space of the clean system. We construct a field
theory on that lattice and apply it to discuss the stability of a weak disorder
(`Wigner-Dyson') and a strong disorder (`Poisson') phase.
