James Webb Young on where ideas come from

This is the way ideas come: after you have stopped straining for them, and have passed through a period of rest and relaxation for the search.

Thus the story about Sir Isaac Newton and his discovery of the law of gravitation is probably not the whole truth. You will remember that when a lady asked the famous scientist how he came to make the discovery he is said to have replied, “By constantly thinking about it.”

It was by constantly thinking about it that he made the discovery possible. But I suspect that if we knew the full history of the case we should find that the actual solution came while he was out taking a walk in the country.

—James Webb Young, A Technique for Producing Ideas