  We consider the holographic prescription problem in a (Lorentzian) AdS
background, deriving from first principles the explicit formulas that relate
the field at infinity with the field in the bulk. In contrast with the previous
studies of the "real-time" holography problem, our derivation uses purely
classical arguments that involve causality, as in the usual treatment of the
holographic prescription problem in Wick-rotated spaces of Euclidean signature.
We show that there is a unique propagator that preserves causality and see that
this provides a simple picture of the relationship between the bulk manifold
and its conformal boundary.
