In order to sustain my theory about gravity (see my work in physics by making a click on October 2009, in 2009 on the right in the “blog archive”, especially the article 9):
“Conjectures and refutations”, chapter 3, part 3, by Karl R. Popper
… This explains that he did very deeply feel the unfinished character of his theory, also the necessity to consider gravity. “That Gravity, writes Newton (See the letter to Richard Bentley, 25th of February 1692-3 (so 1693); see also the letter of the 17th of January.), is innate, inherent and essential in matter, in such a way that a body can act on an other at a distance […] is for me a so huge absurdity that I believe that a person a minimum competent in philosophy will never can fall in this error.”
A bit further:
“Nonetheless, Newton was an essentialist. He did devote some important efforts in order to search for an ultimate explanation for gravity which could be acceptable by trying to deduce the law of the attraction from the hypothesis of a mechanical thrust, only type of causal action admitted by Descartes because the only one which can be explained by the essential propriety of all bodies, extent. But he did not succeed in it. And we can be sure that if he did succeed, he should have considered that his problem did receive its final solution and that he did find the ultimate explanation of gravity…”
But this is true that to think about different ways to explain gravity can help to have some good ideas about other things, even if what is considered is not the final solution for gravity.