  We extend to the Horndeski realm the irreversible thermodynamics description
of gravity previously studied in "first generation" scalar-tensor theories. We
identify a subclass of Horndeski theories as an out-of--equilibrium state,
while general relativity corresponds to an equilibrium state. In this context,
we identify an effective heat current, "temperature of gravity", and shear
viscosity in the space of theories. The identification is accomplished by
recasting the field equations as effective Einstein equations with an effective
dissipative fluid, with Einstein gravity as the equilibrium state, following
Eckart's first-order thermodynamics.
