How did Newton come to postulate the law of inertia when every inertial body that he was considering was (by his own other hypotheses) being acted on by the force of universal gravitation.
In other words, he simultaneously posits the law of Inertia and then theorizes that the contrapositive of his statement is not testable ("If no forces act upon a body at rest, then it will stay at rest", but there is never a situation in which no forces are acting upon a body).
Did he get to this by considering the case of how the inertial velocity of something like a ball being swung on a string in a circle takes off in the direction of the tangent to the circle when the centripetal force is removed, or what were the observations that led to postulate this law? That's the only immediately testable example that I have been able to think of so far, as the same experiment with planets orbiting the sun is not/was not feasible to test in his day.
I'm guessing this law may have some historical context related to the philosophical question of "what makes things move" in Aristotle, and Newton used his law to resolve paradoxes that I am not familiar with.
Or is it something entirely different than what I'm thinking, in that what he actually meant was "it takes less force to keep a body in motion than it does to start a body in motion"?