  The elusive physical nature of Horndeski gravity is elucidated in a new
approach depicting this class of theories as a dissipative effective fluid.
Requiring the constitutive equations of the latter to be those of a Newtonian
fluid restricts the theory to only two disconnected subclasses of "viable"
Horndeski gravity. Therefore, a stress-energy tensor of Horndeski effective
fluid, linear in the first derivatives of the fluid's 4-velocity, is a
sufficient condition for gravitational waves to propagate at light speed. All
other Horndeski theories correspond to exotic non-Newtonian effective fluids.
