  We analyze spacetimes with horizons and study the thermodynamic aspects of
causal horizons, suggesting that the resemblance between gravitational and
thermodynamic systems has a deeper quantum mechanical origin. We find that the
observer dependence of such horizons is a direct consequence of associating a
temperature and entropy to a spacetime. The geometrical picture of a horizon
acting as a one-way membrane for information flow can be accepted as a natural
interpretation of assigning a quantum field theory to a spacetime with
boundary, ultimately leading to a close connection with thermodynamics.
