In his well-known letter to Bentley, Newton writes as follows: “That Gravity should be innate, inherent, and essential to matter, so that one body can act upon another body at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has any philosophical nature or competent faculty of thinking can ever fall into it.”
We also know from his Queries in his book on Optics, that he sought for the explanation of Gravitation in the properties of a subtle, aetherial medium diffused over the universe.
MacLaurin on this point says: “It appears from his letters to Boyle, that this was his opinion early, and if he did not publish his opinion sooner, it proceeded from hence only, that he found he was not able from experiment and observation to give a satisfactory account of this medium, and the manner of its operations in producing the chief phenomena of Nature.”
Therefore, if we accept Newton's suggestion, and endeavour to trace the physical cause of Gravitation in the qualities, properties, and motions of this subtle aetherial medium to which he refers, we shall be simply working on the lines laid down by Sir Isaac Newton himself.
I wish therefore to premise, that the future pages of this work will deal with the hypothesis of this aetherial medium, by which will be accounted for, and that on a satisfactory and physical basis, the universal Law of Gravitation.
Art. 3. Rules of Philosophy.--In order that we may rightly understand the making of any hypothesis, I purpose giving some rules laid down by such philosophers as Newton and Herschel, so that we may be guided by right principles in the development of this new hypothesis as to the cause of Gravitation.
The rules that govern the making of any hypotheses, so far as I can discern, may be summed up under the three following heads--
(1) Simplicity of conception.
(2) Agreement with experience, observation, and experiment.
(3) Satisfactorily accounting for, and explaining all phenomena sought to be explained.