  The first-order thermodynamics of scalar-tensor theory is a novel approach
that exploits the intriguing relationship between gravity and thermodynamics to
better understand the space of gravity theories. It is based on using Eckart's
first-order irreversible thermodynamics on the effective imperfect fluid
describing scalar-tensor gravity and characterises General Relativity as an
equilibrium state, and scalar-tensor theories as non-equilibrium states,
naturally describing the approach to equilibrium. Applications of this
framework to cosmology, extensions to different classes of modified theories,
and the formulation of two complementary descriptions based on the notions of
temperature and chemical potential all contribute to a new and unifying picture
of the landscape of gravity theories.
