  We propose a formulation of the holographic principle, suitable for a
background independent quantum theory of cosmology. It is stated as a
relationship between the flow of quantum information and the causal structure
of a quantum spacetime. Screens are defined as sets of events at which the
observables of a holographic cosmological theory may be measured, and such that
information may flow across them in two directions. A discrete background
independent holographic theory may be formulated in terms of information
flowing in a causal network of such screens. Geometry is introduced by defining
the area of a screen to be a measure of its capacity as a channel of quantum
information from its null past to its null future. We call this a ``weak'' form
of the holographic principle, as no use is made of a bulk theory.
