"So, Newton, what do you think is the cause of Gravity?"
" I haven't got a clue!", Newton replied.
You have to laugh!
This very strange reply is a masterpiece of evasiveness. Newton of course, has been the major theorist on gravity up until Einstin, but he was autistic and very private. In Principia he placed before public scrutiny many of his personal reasonings and revelations, at the behest of Halley. The public and personal as well as private attacks he received as a result almost made him retract his master work from public scrutiny. Much , therefore which he knew, and also much of which he speculated he would not be drawn on.
We find , in the Principia, a magnetic "theory " of gravity, one which was hardly original to Newton, and therefore safe for him to advance . But Newton was also an alchemist, and a keen observer of the behaviours of matter. Thus he knew that magnetism alone could not hold matter together. The one thing that he was careful to avoid was the scandal of witchery! He could not in all conscience be found to be involved with occult practices, therefore his dabbling in Alchemy was methodical and pragmatic.
There was something about matter that made it stick together or combine or crystallise or dissolve, but never attract unless magnetic lodestone was involved. However, Boyle was diligently researching the properties of amber, and finding them not to be magnetic! In his publications on it to friends and the scientific societies, he was at pains to be as vague as he could while encouraging personal exploration. Again the reason was the fear of being condemned as a warlock!
The electric " fluid", that is flowing liquid or gas was posited as inhabiting the interstices of matter . It was therefore a thing to note that Newton assiduously avoids any association with this view of matter at the beginning of Principia. I conclude thst he was aware of these kinds of ideas, but felt it was to incredulous to be anything other than occult powers! Thus his adoption of a magnetic explanation of gravity placed him securely in opposition to any other explanation that was not mechanical, but rather mor Alchemical.
In studying the principles of gravity, Newton takes the spinning top as the bridge between the local almost straight or tangential laws of ballistics, to the clearly vorticular and curved laws in the celestial spheres. The question was, what kept a top spinning on its axis? Newtons general suggestion was a competition between 2 "motives": a centripetal one and a centrifugal one.
This notion came from Newtons spinning bucket experiments. The behaviour of the liquid in a spinning bucket, one on its axis, confirmed Hutgens mechanical model of a point particle bounding around the inside circular wall of the bucket. However imparting that motion involved careful dynamic impulse control. Thus a circular rotation begets a circular rotation .Twisyorque is needed to impart rotation or carefully phased torque, that is an iterative application of Torres is required.
Whether applied internally or externally the observed resultant rotational motion behaved the same on any liquid inside. The liquid began to move radially away from the axis, and begin to mount up against the bucket walls! Newton concluded there must be a centrifugal force involved in rotation, counteracted by a centripetal force.
Magnetism was the only acceptable material property that had both the repulsive and attractive forces necessary for gravitational rotation.
Newton also learned about the tidal effect of rotation from his bucket experiments.