Dear Sir Isaac Newton,
Thank you for your letter. I have a request of you, could you kindly explain your theory of gravitation?
My theory of gravity is quite simple:
All objects exert a force upon all other objects in the universe. The size of this force is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
To determine the distance between two objects is to divide the mass of each by the force each object exerts on the other.
Gravity is just a consequence of the inverse square law. It tells us how small objects orbit a larger one, but it doesn't tell us how large bodies move in response to forces exerted by smaller bodies. To do that, we need Newton's Second Law of Motion: F = ma.
That's how I saw it. When I used this formula, I discovered that the law of gravitation and all its wonders sprang from it.