At the beginning of 1665, when Isaac Newton was twenty-three years old, he returned to his native village for a period of two years to escape the plague that had closed down Cambridge University. He later wrote that these years were his most fruitful and creative, and recalls in particular that in 1666 he developed the integral calculus, experimentally verified the composite nature of light, and refined his gravitational theory to the point that he was able to satisfy himself through calculation that the earth's gravity holds the moon in orbit. Recent Newtonian scholarship has effectively called his memory into question somewhat on these points, but the marvelous year of 1666 may surely be taken as symbolic of a decisive turning point in the history of human thought.