Newton's Prime Age of Invention  (reference link)

In the beginning of the year 1665 I found the Method of approximating series & the Rule for reducing any dignity of any Binomial into such a series. The same year in May I found the method of Tangents of Gregory & Slusius, & in November had the direct method of fluxions & the next year in January had the Theory of Colours & in May following I had entrance into y^e inverse method of fluxions. And the same year I began to think of gravity extending to y^e orb of the Moon & (having found out how to estimate the force with w^ch [a] globe revolving within a sphere presses the surface of the sphere) from Keplers rule of the periodical times of the Planets being in sesquialterate proportion of their distances from the center of their Orbs, I deduced that the forces w^ch keep the Planets in their Orbs must [be] reciprocally as the squares of their distances from the centers about w^ch they revolve: & thereby compared the force of gravity at the surface of the earth, & found them answer pretty nearly. All this was in the two plague years 1665-1666. For in those days I was in the prime of my age for invention & minded Mathematicks & Philosophy more then at any time since.