Many of us may think (or by not thinking, assume) that Gravity is some intrinsic quality of Matter, but many theoretical physicists throughout history would disagree with this view. Newton, for one, wrote in a letter to an acquaintance that Gravity is not at all “innate, inherent, and essential to Matter.” For Newton, Matter innately or absolutely possessed only: Inertia (aka Mass), Hardness (this possibly could be viewed as Density), and Extension (the three Dimensions). Others, such as Descartes and Spinoza, thought the only quality that Matter absolutely possessed was Extension (and Descartes barely thought this, for the “atoms” of his “Matter” were no more than vortexes in the Ether).
Before mainstream physics ditched the idea of the Ether in the early 1900s, many physicists (those who gave any thought at all to the question, “WHY Gravity?”) thought that the existence of both Ether and Matter gave balance to the Universe. This is because Ether was seen by many as being made of subtle corpuscles which repel one another– somewhat akin to how the charge of electrons cause them to keep their distance from each other. On the other hand, Matter tends to attract other Matter (there were several theories as to why). Matter needs the Ether for balance, or else, as Heimann quotes Stephen Hales as saying in Concepts Of Ether (edited by G.N. Cantor and M.J.S. Hodge), the Universe would eventually become “one inactive cohering lump.” Without the self-repulsive effects of the Ether, “Gravitation,” says James Hutton, “would soon bring all the matter of this machine to rest.”
Now… as to why Matter coheres… there’s more than one line of thought on this. Some thought of Matter as being immersed, as Lavoiser put, in a counter-attracting fluid (the Ether). Riemann, Lavoiser, and Cullen (like Newton) thought that Ether inside of Matter is rarefied, or partially displaced, by the molecules of Matter, and therefore the density of the Ether is greater outside of Matter than inside. The attempt of Ether to equalize its “pressure” (if we can speak of Ether “pressure”) is what causes objects to be pushed toward each other. Cullen described the field of Gravity around an object as the density gradient of the “atmosphere” of Ether around it.
Cullen’s additional theoretical contribution was to suggest that what keeps an individual object together is the pressure of the Ether against the object’s own molecules. The Ether is out there repulsing itself, and as a side-effect, keeps pushing together these clumps of matter.
Landon tells us that George LeSage had a shield theory of Gravity. In this view, we imagine the Ether as streaming equally through the Universe in every direction. To some extent, the Ether will even stream through Matter– but not without some obstruction. It is Matter’s obstruction of the Ether streams that gives rise to Gravity. The Ether that does not pass straight through the Matter will impact it.
If there were only one object around, the Ether would impact it equally from all sides, and the object would not move since no direction would be favored. However, if we introduce a second object, this second object will act as a shield to the first, partially protecting it from Ether streams coming from the direction of the second object. Additionally, the first object will be acting as a partial shield to the second, giving it some cover from the Ether that is streaming from the opposite direction.
Thus, the two objects will be impacted less by Ether from their facing sides. The unequal bombardment of Ether will then cause the objects to be pushed toward each other.