How a falling apple could have helped Newton discover universal gravity

The article delves into the intriguing disagreement among historians of science regarding the origin of the idea of universal gravity. Prominent researchers of the genesis of Newton's Principia did not believe that he could have come up with this idea in 1666, after observing a falling apple, as he claimed. On the other hand, prominent biographers of Newton did believe. The proposed key to solving this paradox is a thought experiment described in Newton's manuscript, 'The System of the World,' which preceded the creation of his 'Principia.' According to the proposed 'subjunctive' scenario, it was possible to come from the motion of a projectile to the concept of universal gravity, relying only on the knowledge and skills of Galileo in 1611.