Theoreticians have formulated a set of fundamental criteria that any theory of gravity should satisfy, two purely theoretical and two that are based on experimental evidence. Thus, a theory must be complete (capable of analyzing from the "first principles" the result of any experiment of interest), self-consistent (its prediction for the outcome of each experiment must be unique), relativistic (at the limit when gravity is neglected compared to other physical interactions, non-gravitational laws of physics must be reduced to special relativity laws), and with the correct Newtonian limit (within the limits of weak gravitational fields and slow motions, they must reproduce Newton's laws). DOI: 10.13140/RG.2.2.27848.2688