score:31
I believe the authors are paraphrasing a fairly well-known extract from a draft of a letter from Newton to Pierre Des Maizeaux, written in 1718. The extract in full reads as follows:
In the beginning of the year 1665 1 found the method of approximating series and the rule for reducing any dignity [power] of any binomial into such a series. The same year in May I found the method of tangents of Gregory and Slusius, and in November had the direct method of fluxions and the next year [1666] in January had the theory of colours and in May following I had entrance into the inverse method of fluxions. And the same year I began to think of gravity extending to the orb of the moon ... All this was in the two plague years of 1665 and 1666, for in those days I was in the prime of my age for invention and minded Mathematics and Philosophy more than at any time since.
(my emphasis)
The original draft of the letter is held in the collection of Cambridge University Library.
It has been digitised and is available to view under the reference MS Add.3968.29: 420-437 from their online collection.