Criticism: Newton objected to the theory because drag must lead to noticeable deviations of the orbits which were not observed.[9] Another problem was that moons often move in different directions, against the direction of the vortex motion. Also, Huygens' explanation of the inverse square law is circular, because this means that the aether obeys Kepler's third law. But a theory of gravitation has to explain those laws and must not presuppose them.[6][9]