The centripetal force is, however, the exact counterpart and complement of the centrifugal force, in the same way that the latter is the exact counterpart of the former, as we have already learned that the centrifugal force operates along the same path, and that it is subject to the same law of proportion, being equal to the product of the masses of a body (Art. 85), and further, that its intensity is inversely as the square of the distance (Arts. 66, 74 and 84). We have, however, discovered that the physical cause of the centrifugal force is due to the pressure of the electro-magnetic Aether (Art. 96). If, therefore, the physical cause of that law which is the complement and counterpart of Gravitation Attraction is to be found, and alone found, in the pressure and motions of the electro-magnetic Aether, then it is only logical and reasonable to infer that the physical cause of the Attraction of Gravitation is to be found in the same electro-magnetic Aether. That the Attraction of Gravitation is to be found in this medium is now an accepted hypothesis among scientists. Because, unless the physical cause of Gravitation Attraction is to be found in this electro-magnetic Aether, then, in order to account for that attraction, we should have to postulate the existence of another medium in space, in lieu of the Aether, which would be to the centripetal force what the Aether is to the centrifugal force. This would be distinctly unphilosophical, as it would be a violation of the first two rules of our philosophy, in that it would not be simple in its conception, and that such a hypothesis would imply the existence of two media which would occupy the same planetary and interstellar space at one and the same time, and this is a violation of all experience in its widest form. Therefore the physical cause of Gravitation must be sought for in the same medium which gives the physical cause of the companion force, and that medium is the electro-magnetic Aether. Professor Preston in his _Theory of Light_ is of this opinion. In Art. 327 he writes: "To account for the propagation of heat and light, that is, of radiant energy, we have postulated the existence of a medium filling all space. But the transference of the energy of radiant heat and light is not the only evidence we have in favour of the existence of an Aether. Electric, magnetic and electro-magnetic phenomena and Gravitation itself point in the same direction." Professor Lodge, in his _Modern Views of Electricity_, is even more explicit in his statement of the case. On page 338 he states: "Gravitation is explainable by differences of pressure in the medium (_i. e._ the Aether) caused by some action between it and matter not yet understood." Further, Newton himself suggested that the physical cause of Gravitation Attraction was to be found in that aetherial medium which pervaded all space.