How did Isaac Newton discover the first law of motion

Newton worked in many areas of mathematics and physics. He developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in 1686, he presented his three laws of motion in the "Principia Mathematica Philosophiae Naturalis

it is believed that he first started studying the effects of gravity after watching a apple fall. Why did it fall, and what determined the speed at which it fell? it is believed that this incident, as well as his curiosity for seeing stars and planets above without them falling to the ground, led him to develop the laws of motion.